Dumb (4)
I believe the answer is:
mute
'dumb' is the definition.
(I know that dumb can be written as mute)
This is all the clue.
(Other definitions for mute that I've seen before include "Speechless, soundless" , "Unspeaking" , "Device that softens the tone of a musical instrument" , "having no voice" , "Silencer" .)