Low in volume (4)
I believe the answer is:
soft
'low in volume' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for soft that I've seen before include "Yielding, easily cut" , "(Of drink) non-alcoholic" , "Gentle; non-alcoholic" , "Not hard, rough or harsh" , "Yielding; quiet" .)