Not sharp, not flat, not affected (7)
I believe the answer is:
natural
This clue is a triple definition!
'not sharp' is the first definition.
('natural' is the opposite to 'sharp')
'not flat' is the second definition.
('natural' is the opposite to 'flat')
'not affected' is the third definition.
('natural' is the opposite to 'affected')
(Other definitions for natural that I've seen before include "Unaffected; off-white" , "Not made; off-white" , "Unaffected; unaltered" , "Not man-made; off-white" , "one short of being sharp?" .)