Religious group protecting most prudish offender (7)
I believe the answer is:
culprit
'offender' is the definition.
(culprit is a kind of offender)
'religious group protecting most prudish' is the wordplay.
'religious group' becomes 'cult' (I've seen this before).
'protecting' is an insertion indicator.
'most' means to remove the last letter.
'prudish' becomes 'prim' (synonyms).
'prim' with its final letter taken off is 'pri'.
'cult' enclosing 'pri' is 'CULPRIT'.
(Other definitions for culprit that I've seen before include "he should have conviction" , "Person responsible for wrongdoing" , "The one to blame, guilty party" , "Offender or guilty party" , "The person to blame, the guilty one" .)