Flat's counterpart (5)
I believe the answer is:
sharp
'flat's counterpart' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sharp that I've seen before include "British musician" , "Keen - exactly" , "Fly - biting" , "Clear-cut" , "Musical notation of a half step higher than the note named" .)