Unspeaking (4)
I believe the answer is:
mute
'unspeaking' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mute that I've seen before include "having no voice" , "Device for softening musical sound" , "Speechless, soundless" , "Silencer" , "Device that softens the tone of a musical instrument" .)