Old books, I see, are unusual objects (7)
I believe the answer is:
exotica
'unusual objects' is the definition.
(I've seen this in another clue)
'old books i see are' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'books' becomes 'OT' (abbreviation for Old Testament).
'see' becomes 'C' (the word for the letter, according to Chambers).
'are' becomes 'a' (short for 'are', historical unit of measurement).
'ex'+'ot'+'i'+'c'+'a'='EXOTICA'
(Other definitions for exotica that I've seen before include "Collection of foreign objects" , "Strange, galmorous things from abroad" , "Objects from afar" , "Foreign or unfamiliar things" , "Unusual or foreign items" .)