Whim of a musician? (8)
I believe the answer is:
crotchet
'whim' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for crotchet that I've seen before include "Quarter of a semibreve" , "One in bar" , "Musical note equal to half a minim" , "Two quavers as one note" , "as mark scored" .)