Least loud (7)
I believe the answer is:
softest
'least loud' is the definition.
('soft' is the opposite of 'loud' so 'softest' should be a synonym for 'least loud')
This is the entire clue.
(Other definitions for softest that I've seen before include "Lowest in volume" , "Most delicate or most lenient" , "Least hard or rough" , "most weak" .)