Musicians forbidden to speak (4)
I believe the answer is:
band
'musicians' is the definition.
(I've seen this before)
'forbidden to speak' is the wordplay.
'forbidden' becomes 'banned' (banning is a kind of forbidding).
'to speak' shows a homophone (sound like).
'banned' sounds like 'BAND'.
(Other definitions for band that I've seen before include "Zone; group" , "company" , "Musicians - circle" , "Loop; rock group" , "Belt; group of musicians" .)