Books in excellent political opponent (4)
I believe the answer is:
anti
'political opponent' is the definition.
'anti' can be an answer for 'opponent' (someone opposed to something). I am unsure of the 'political' bit.
'books in excellent' is the wordplay.
'books' becomes 'nt' (abbreviation for New Testament).
'in' means one lot of letters goes inside another.
'excellent' becomes 'ai' (resembles 'An', old-fashioned term for excellent).
'nt' placed within 'ai' is 'ANTI'.
Can you help me to learn more?
(Other definitions for anti that I've seen before include "One who is against (something)" , "one opposing?" , "Policy's opponent" , "Someone opposed (to a policy?)" , "Not favouring" .)