Sound-off button (4)
I believe the answer is:
mute
'sound-off button' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mute that I've seen before include "Device for softening musical sound" , "Silent - dumb" , "Speechless, soundless" , "Violin clamp; type of swan" , "Deaden sound of" .)