Strengthen check on police? (9)
I believe the answer is:
reinforce
'strengthen' is the definition.
(I know that reinforce is a type of strengthen)
'check on police?' is the wordplay.
'check' becomes 'rein' (both can mean to control or limit).
'on' means one lot of letters go next to another.
'police?' becomes 'force' (the force can refer to the police force).
'rein'+'force'='REINFORCE'
(Other definitions for reinforce that I've seen before include "Buttress" , "Strengthen or support with confrerie" , "Make stronger" , "provide support" , "Give added strength to" .)