Least quiet (8)
I believe the answer is:
noisiest
'least quiet' is the definition.
('noisy' is the opposite of 'quiet' so 'noisiest' should be a synonym for 'least quiet')
This is the entire clue.
(Other definitions for noisiest that I've seen before include "Making the loudest sounds" , "Making the most sound" , "Most rowdy" , "Making the most loud sounds" , "Biggest racket" .)