Books included in agreement with canine association (6)
I believe the answer is:
dental
'with canine association' is the definition.
(canine is a type of tooth)
'books included in agreement' is the wordplay.
'books' becomes 'NT' (New Testament).
'included in' is an insertion indicator.
'agreement' becomes 'deal' (deal is a kind of agreement).
'nt' inserted into 'deal' is 'DENTAL'.
(Other definitions for dental that I've seen before include "Consonant" , "Concerning teeth" , "surgery" , "sort of surgeon" , "Relating to teeth" .)