A note of whimsy? (8)
I believe the answer is:
crotchet
I believe this clue is a double definition.
'a note' is the first definition.
(musical note)
'whimsy?' is the second definition.
(thesaurus)
'of' is the link.
(Other definitions for crotchet that I've seen before include "A note in music equal to half a minim" , "Two quavers as one note" , "Musical note equal to half a minim" , "as mark scored" , "Quarter of a semibreve" .)