Drive? Sadly, time's up (7)
I believe the answer is:
impetus
'drive?' is the definition.
(impetus is a drive to do something)
'sadly time's up' is the wordplay.
'sadly' is an anagram indicator.
'times'+'up'='timesup'
'timesup' is an anagram of 'IMPETUS'.
(Other definitions for impetus that I've seen before include "Driving force, forward movement" , "Incentive - time's up (anag)" , "influence" , "Catalyst" , "Thrust" .)