Strengthen and control by coercion (9)
I believe the answer is:
reinforce
'strengthen' is the definition.
(I know that reinforce is a more specific form of the action strengthen)
'control by coercion' is the wordplay.
'control' becomes 'rein' (reining is a kind of controlling).
'by' is a charade indicator (letters next to each other).
'coercion' becomes 'force' (I've seen this in another clue).
'rein'+'force'='REINFORCE'
'and' is the link.
(Other definitions for reinforce that I've seen before include "give added support" , "Make stronger" , "increase support" , "Give added strength to" , "provide support" .)