Under restraint, police officers toughen up (9)
I believe the answer is:
reinforce
'officers toughen up' is the definition.
I can't judge whether this defines the answer.
'under restraint police' is the wordplay.
'under' says to put letters next to each other.
'restraint' becomes 'rein' (I've seen this in another clue).
'police' becomes 'force' (the force can refer to the police force).
'rein' next to 'force' is 'REINFORCE'.
Can you help me to learn more?
(Other definitions for reinforce that I've seen before include "Strengthen or support with confrerie" , "Buttress" , "give added support" , "Make stronger" , "increase support" .)