Books in excellent opponent (4)
I believe the answer is:
anti
'opponent' is the definition.
(I've seen this before)
'books in excellent' is the wordplay.
'books' becomes 'nt' (New Testament).
'in' is an insertion indicator.
'excellent' becomes 'ai'.
'nt' put inside 'ai' is 'ANTI'.
(Other definitions for anti that I've seen before include "Policy's opponent" , "Prefix meaniing against" , "one opposing?" , "(Person who is) not in favour" , "[NO MAN]" .)