Discipline children with a gun! (7)
I believe the answer is:
chasten
'discipline' is the definition.
(I've seen this before)
'children with a gun' is the wordplay.
'children' becomes 'ch' (genealogical abbreviation).
'with' says to put letters next to each other.
'gun' becomes 'sten' (British sub-machine gun).
'ch'+'a'+'sten'='CHASTEN'
(Other definitions for chasten that I've seen before include "Punish, purify" , "Suppress" , "Moderate" , "Have a restraining effect on (someone)" , "Castigate or punish" .)