Book is indeed diminished (6)
I believe the answer is:
dented
'diminished' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
'book is indeed' is the wordplay.
I cannot really understand how this works, but
'book' could be 'nt' (New Testament) and 'nt' is present in the answer.
The remaining letters 'deed' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for dented that I've seen before include "made an impression" , "Bashed in" , "Dinged" , "Made small depression in metal surface" , "Tended to be depresed" .)