
<PAGE>   1
                                                                 EXHIBIT 5.1

                          [SIDLEY & AUSTIN LETTERHEAD]


                              January  14, 1997


CLARCOR Inc.
2323 Sixth Street
P.O. Box 7007
Rockford, Illinois 61125

                   Re:  Registration of 1,209,302 Shares of Common Stock
                        and Associated Preferred Stock Purchase Rights

Ladies and Gentlemen:

          We have acted as counsel to CLARCOR Inc., a Delaware corporation (the
"Company"), in connection with the preparation and filing with the Securities
and Exchange Commission (the "Commission") under the Securities Act of 1933, as
amended (the "Securities Act"), of the Company's registration statement on Form
S-4 (the "Registration Statement") relating to the registration of 1,209,302
shares of common stock, $1 par value, of the Company ("Company Common Stock"),
together with 1,209,302 Series B Junior Participating Preferred Stock Purchase
Rights (the "Rights") associated therewith, to be issued pursuant to the terms
of the Agreement and Plan of Merger dated as of September 23, 1996 (the "Merger
Agreement") among the Company, CUAC Inc., an Ohio corporation and a wholly-owned
subsidiary of the Company ("Sub"), and United Air Specialists, Inc., an Ohio
corporation ("UAS"), which provides for the merger (the "Merger") of Sub with
and into UAS, with UAS surviving as a wholly-owned subsidiary of the Company.
Pursuant to the Merger Agreement:  (a) each share of common stock, without par
value, of UAS ("UAS Common Stock") outstanding immediately prior to the
effective time of the Merger will be converted into a fractional share of
Company Common Stock (the shares of Company Common Stock to be issued in the
Merger being hereinafter referred to as the "Merger Shares"); and (b) each 
option to purchase UAS Common Stock granted under a UAS Stock Plan (as defined
in the Merger Agreement) which is outstanding immediately prior to the
Effective Time will become an   option (a "Substitute Option") to purchase
shares of Company Common Stock (the "Stock Option Shares").  The Merger Shares
and the Stock Option Shares are hereinafter sometimes referred to collectively
as the "New Shares."  The terms of the Rights are set forth in the Stockholder
Rights Agreement dated as of March 28, 1996 (the "Rights Agreement"), between
the Company and The First Chicago Trust Company of New York, as Rights Agent.

          For the purpose of rendering the opinions expressed below, we have 
reviewed the Merger Agreement, the Registration Statement and the Exhibits to
the latter.  We have also examined the originals, or copies of originals
certified or otherwise identified to our 

<PAGE>   2
CLARCOR Inc.
January 14, 1997
Page 2


satisfaction, of the corporate records of the Company and of
such other agreements, documents, instruments and certificates of public 
officials, officers and representatives of the Company and other persons, have
examined such questions of law and have satisfied ourselves as to such matters
of fact as we have deemed relevant and necessary as a basis for the opinions
expressed below. We have assumed the authenticity of all documents submitted to
us as originals, the genuineness of all signatures, the legal capacity of all
natural persons and the conformity with the original documents of any copies
thereof submitted to us for our examination. 

          Based on the foregoing, it is our opinion that:

          1. The Company is duly incorporated and validly existing under the
laws of the State of Delaware.

          2. Each Merger Share will be legally issued, fully paid and
non-assessable when: (i) the Registration Statement, as finally amended, shall
have become effective under the Securities Act; and (ii) the Merger shall have
become effective under the General Corporation Law of the State of Ohio.

          3. Each Stock Option Share will be legally issued, fully paid and
non-assessable when: (i) the Registration Statement, as finally amended, shall
have become effective under the Securities Act; (ii) the Merger shall have
become effective under the General Corporation Law of the State of Ohio, and
(iii) a certificate representing such Stock Option Share shall have been duly
executed, countersigned and registered and duly delivered upon receipt of the
agreed consideration therefor (not less than the par value thereof) determined
in accordance with the terms of the written agreement applicable to the related
Substitute Option.

          4. Each Right associated with a New Share will be legally issued when:
(i) such Right shall have been duly issued in accordance with the terms of the
Rights Agreement; and (ii) the associated Merger Share or Stock Option Share
shall have been duly issued as set forth in paragraph 2 or 3, respectively.

          The foregoing opinions are limited to the federal laws of the United
States of America and the General Corporation Law of the State of Delaware.  We
express no opinion as to the application of the securities or blue sky laws of
the various states to the issuance or sale of the New Shares or the associated
Rights.

          We hereby consent to the filing of this opinion as an Exhibit to the
Registration Statement and to all references to our firm included in or made
part of the Registration Statement.

                                     Very truly yours,


                                      SIDLEY & AUSTIN





