One who prohibits flag? (6)
I believe the answer is:
banner
This clue is a double definition.
'one who prohibits' is the first definition.
(person who bans?)
'flag?' is the second definition.
(I know that banner is a type of flag)
(Other definitions for banner that I've seen before include "Flag used as standard" , "protester's statement?" , "Web page header; flag" , "Website advert; flag" , "Distinctive flag" .)