Subdue the child that has a gun (7)
I believe the answer is:
chasten
'subdue' is the definition.
(synonyms)
'the child that has a gun' is the wordplay.
'the child' becomes 'ch' (abbreviation for child).
'that has' 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" , "Subdue the pride of, discipline" , "Subdue - a stench (anag)" , "Subdue pride of with a stench" , "Moderate" .)