Listening to pop group was forbidden (6)
I believe the answer is:
banned
'forbidden' is the definition.
(banning is a kind of forbidding)
'listening to pop group' is the wordplay.
'listening to' indicates a 'sounds like' (homophone) clue.
'pop group' becomes 'band' (I've seen this before).
'band' is a homophone of 'BANNED'.
'was' is the link.
(Other definitions for banned that I've seen before include "Taboo" , "expelled" , "Legally prohibited" , "Prohibited by censor" , "Barred" .)