Sudden whim (7)
I believe the answer is:
impulse
'sudden whim' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for impulse that I've seen before include "Whim; thrust" , "Miles up (anag)" , "A sudden urge or whim" , "I may cause desire" , "Sudden urge to do something" .)