Capricious idea (6)

I believe the answer is:

whimsy

(Other definitions for whimsy that I've seen before include "Capricious idea - sounds like Sayers' sleuth" , "Quaint - odd" , "Capricious humour" , "Quaint fanciful humour" , "Fanciful thoughts" .)

I've seen this clue in the Evening Standard.

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