Gangster, they say, forbad the action (6)
I believe the answer is:
bandit
'gangster' is the definition.
(thesaurus)
'they say forbad the action' is the wordplay.
'they say' shows a homophone (sound like).
'forbad' becomes 'banned' (banning is a kind of forbidding).
'the action' becomes 'it' (I can't explain this - if you can you should give a lot more credence to this answer).
'banned' sounds like 'band'.
'band'+'it'='BANDIT'
Can you help me to learn more?
(Other definitions for bandit that I've seen before include "Dan bit the robber badly" , "Armed brigand" , "Highwayman" , "Thief" , "hood" .)