Weird flat, perhaps, for musician? (9)
I believe the answer is:
unnatural
This clue is a double definition.
'weird' is the first definition.
(thesaurus)
'flat perhaps for musician?' is the second definition.
(in music, sharps and flats are not naturals)
(Other definitions for unnatural that I've seen before include "Sharp or flat" , "Forced" , "Annual rut may be abnormal" , "Monstrous" , "Freakish" .)