Discipline for having taken a gun to church (7)
I believe the answer is:
chasten
'discipline' is the definition.
(I've seen this before)
'having taken a gun to church' is the wordplay.
'having taken' is an insertion indicator.
'gun' becomes 'sten' (British sub-machine gun).
'to' means one lot of letters go next to another (I've seen this in other clues).
'church' becomes 'ch' (common abbreviation).
'sten' put after 'ch' is 'chsten'.
'a' placed within 'chsten' is 'CHASTEN'.
'for' acts as a link.
(Other definitions for chasten that I've seen before include "Punish, purify" , "Subdue the pride of, discipline" , "Castigate or punish" , "Moderate" , "Subdue, correct by punishing" .)