Fanciful idea (4)
I believe the answer is:
whim
'fanciful idea' 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 "Sudden or capricious idea" , "Urge" , "Fanciful idea" , "Sudden odd desire" , "Caprice, sudden fancy" .)