Time's up disastrously for Momentum (7)
I believe the answer is:
impetus
'for momentum' is the definition.
(impetus can mean momentum or motivation)
'time's up disastrously' is the wordplay.
'disastrously' indicates an anagram (I've seen this in other clues).
'times'+'up'='timesup'
'timesup' with letters rearranged gives 'IMPETUS'.
(Other definitions for impetus that I've seen before include "Motivation" , "Thrust" , "Incentive, stimulus" , "Drive" , "Imputes to force with which a body moves" .)