Silence, quieten (4)
I believe the answer is:
mute
This clue is a double definition.
'silence' is the first definition.
(muting is a kind of silencing)
'quieten' is the second definition.
(muting is a kind of quietening)
(Other definitions for mute that I've seen before include "One refuses to answer" , "Silent, refraining from speaking" , "Violin clamp; type of swan" , "Silencer" , "Deaden sound of" .)