Sign carried by the demonstrators that he won’t allow? (6)
I believe the answer is:
banner
'sign carried by the demonstrators that' is the definition.
'banner' can be an answer for 'sign' (thesaurus). I am unsure of the rest of the definition.
'he won't allow?' is the wordplay.
'he' becomes 'Ban'.
'won't allow?' becomes 'ner' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ban'+'ner'='BANNER'
Can you help me to learn more?
(Other definitions for banner that I've seen before include "Large fabric sign" , "Flag with a slogan" , "Website advert; flag" , "Standard; heading" , "Flag; headline" .)