Check the power of support? (9)
I believe the answer is:
reinforce
'support?' is the definition.
(supporting is a kind of reinforcing)
'check the power' is the wordplay.
'check' becomes 'rein' (both can mean to control or limit).
'the power' becomes 'force' (force is a kind of power**).
'rein'+'force'='REINFORCE'
'of' is the link.
(Other definitions for reinforce that I've seen before include "give added support" , "increase support" , "Buttress" , "provide support" , "Make stronger" .)