Tune that's fine but not loud (3)
I believe the answer is:
air
'tune' is the definition.
('air' can be a synonym of 'tune')
'fine but not loud' is the wordplay.
'fine' becomes 'fair' (I've seen this before).
'but not' is a deletion indicator.
'loud' becomes 'f' (A musical term).
'fair' with 'f' taken out is 'AIR'.
'that's' acts as a link.
(Other definitions for air that I've seen before include "Vent; atmosphere" , "Light wind; song" , "Broadcast; express" , "Look - warm and dry" , "Mixture of gases, especially oxygen" .)