They forbid the use of flags (7)
I believe the answer is:
banners
'flags' is the definition.
(banner is a kind of flag)
'they forbid the use' is the wordplay.
I cannot really understand how this works, but
'forbid' could be 'ban' (banning is a kind of forbiding) and 'ban' is found within the answer.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for banners that I've seen before include "Long strips with messages hung in public places" , "Newspaper headlines or flags" .)