Prisoner accepts a new church law (5)
I believe the answer is:
canon
'church law' is the definition.
(I've seen this in another clue)
'prisoner accepts a new' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'accepts' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'a'+'n'='an'
'con' enclosing 'an' is 'CANON'.
(Other definitions for canon that I've seen before include "Part of chapter" , "Such literary works are genuine" , "General rule; clergyman" , "Body of generally established principles for a given context" , "priest for cathedral duties" .)