Cause pain to stern arresting French cop (7)
I believe the answer is:
afflict
'cause pain to' is the definition.
(I've seen this before)
'stern arresting french cop' is the wordplay.
'stern' becomes 'aft' ('aft' can be similar in meaning to 'stern').
'arresting' is an insertion indicator.
'french cop' becomes 'flic' (I've seen this before).
'aft' placed around 'flic' is 'AFFLICT'.
(Other definitions for afflict that I've seen before include "Cause pain to" , "Vex" , "Painfully affect" , "Beset" , "Cause distress to" .)