Criminal, say, heads for priest, really intending to enter religious group (7)
I believe the answer is:
culprit
'criminal' is the definition.
(I've seen this before)
'heads for priest really intending to enter religious group' is the wordplay.
'heads for' suggests taking the first letters (I've seen 'heading for' mean this).
'to enter' indicates putting letters inside.
'religious group' becomes 'cult' (I've seen this before).
The first letters of 'priest really intending' is 'pri'.
'pri' going into 'cult' is 'CULPRIT'.
'say' is the link.
(Other definitions for culprit that I've seen before include "Person responsible for wrongdoing" , "one awaiting trial" , "Perpetrator of a wrongdoing" , "he should have conviction" , "One perpetrating crime" .)