Flight of fancy (6)
I believe the answer is:
whimsy
'flight of fancy' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for whimsy that I've seen before include "Quaint fanciful humour" , "Gentle fantasy" , "Capricious humour" , "Playfully quaint behaviour" , "Something fanciful" .)