Sudden impulse (4)
(Other definitions for urge that I've seen before include "Encourage to proceed, drive onward" , "Try hard to persuade, advise earnestly" , "Plead" , "Compulsion; egg on" , "Constrain" .)
'sudden impulse' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for whim that I've seen before include "Urge" , "Caprice, sudden fancy" , "Fad" , "Sudden or capricious idea" , "Fanciful impulse" .)