Increase support to control violence (9)
I believe the answer is:
reinforce
'increase support' is the definition.
(I've seen this before)
'control violence' is the wordplay.
'control' becomes 'rein' (reining is a kind of controlling).
'violence' becomes 'force' (synonyms).
'rein'+'force'='REINFORCE'
'to' acts as a link.
(Other definitions for reinforce that I've seen before include "Buttress" , "Strengthen or support with confrerie" , "Give added strength to" , "give added support" , "provide support" .)