Strengthen restraint by army (9)
I believe the answer is:
reinforce
'strengthen' is the definition.
(I know that reinforce is a more specific form of the action strengthen)
'restraint by army' is the wordplay.
'restraint' becomes 'rein' (I've seen this in another clue).
'by' says to put letters next to each other.
'army' becomes 'force' (I've seen this before).
'rein'+'force'='REINFORCE'
(Other definitions for reinforce that I've seen before include "Make stronger" , "provide support" , "Buttress" , "give added support" , "Give added strength to" .)