It's silent, stupid! (4)
I believe the answer is:
dumb
I believe this clue is a double definition.
'it's silent' is the first definition.
(I've seen this before)
'stupid' is the second definition.
('dumb' can be similar in meaning to 'stupid')
(Other definitions for dumb that I've seen before include "Struck with amazement" , "Mute" , "Incapable of speech" , "Thick" , "Voiceless" .)