A musical, but not a book (4)
I believe the answer is:
acts
'book' is the definition.
(book of the Bible)
'a musical but not a' is the wordplay.
'musical' becomes 'Cats' (Andrew Lloyd Webber musical).
'but not' suggests deleting specific letters.
'cats' with 'a' removed is 'cts'.
'a'+'cts'='ACTS'
(Other definitions for acts that I've seen before include "book form" , "Cast in major divisions of plays, like Shakespeare's" , "Is effective" , "Behaves or plays" , "Performs on stage" .)