Setter's potentially upset Momentum (7)
I believe the answer is:
impetus
'momentum' is the definition.
(I've seen this in another clue)
'setter's potentially upset' is the wordplay.
'setter's' becomes 'im' (referring to the person who set the crossword).
'potentially' indicates anagramming the letters (letters have the potential to spell a new word).
'upset' anagrammed gives 'petus'.
'im'+'petus'='IMPETUS'
(Other definitions for impetus that I've seen before include "Drive" , "Thrust" , "Incentive - time's up (anag)" , "influence" , "Moving force" .)