Sudden impulse (4)

I think it could be either:

urge

whim

'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" .)

I've seen this clue in The Times, The Guardian, The Telegraph and the LA Times.

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