Unforced, one may cancel a flat (7)
I believe the answer is:
natural
This clue is a double definition.
'unforced' is the first definition.
(I've seen this before)
'one may cancel a flat' is the second definition.
(natural musical note as opposed to a flat or sharp)
(Other definitions for natural that I've seen before include "chemical-free" , "white key on a piano?" , "Unaffected, normal in manner" , "lacking sophistication" , "Fuel from underground" .)