Group says it's forbidden (4)
I believe the answer is:
band
'group' is the definition.
(band can mean a group of people)
'says it's forbidden' is the wordplay.
'says' shows a homophone (sound like).
'it's forbidden' becomes 'banned' (banning is a kind of forbidding).
'banned' sounds like 'BAND'.
(Other definitions for band that I've seen before include "Body" , "Pop group, perhaps" , "Outline drawing" , "Ring or musical group (4)" , "Capital of Tibet" .)