Free snifters later — willpower's required! (4-9)
I believe the answer is:
self-restraint
'willpower's required' is the definition.
(will-power is required for self-restraint)
'free snifters later' is the wordplay.
'free' indicates an anagram.
'snifters'+'later'='snifterslater'
'snifterslater' with letters rearranged gives 'SELF-RESTRAINT'.
(Other definitions for self-restraint that I've seen before include "Personal control" , "Refraining from excess" , "Control of one's own actions" .)