Possibly time's up for Momentum (7)
I believe the answer is:
impetus
'momentum' is the definition.
(I've seen this before)
'possibly time's up' is the wordplay.
'possibly' indicates anagramming the letters.
'times'+'up'='timesup'
'timesup' anagrammed gives 'IMPETUS'.
'for' is the link.
(Other definitions for impetus that I've seen before include "Motivation" , "Driving force, momentum" , "Catalyst" , "Thrust" , "influence" .)