Outlaw, proscribe (6)
I believe the answer is:
forbid
This clue is a double definition.
'outlaw' is the first definition.
(I've seen this before)
'proscribe' is the second definition.
(synonyms)
(Other definitions for forbid that I've seen before include "Refuse permission, order not to" , "Interdict" , "Prohibit, veto" , "bar" , "Deny, prohibit" .)