Strengthen the power of restraint (9)
I believe the answer is:
reinforce
'strengthen' is the definition.
(I know that reinforce is a type of strengthen)
'power of restraint' is the wordplay.
I cannot quite understand how this works, but
'power' could be 'force' (force is a kind of power) and 'force' is found within the answer.
'restraint' could be 'rein' (I've seen this before) and 'rein' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'the' is the link.
Can you help me to learn more?
(Other definitions for reinforce that I've seen before include "provide support" , "increase support" , "Strengthen or support with confrerie" , "Buttress" , "give added support" .)