One ruling against what demonstrators carry? (6)

I believe the answer is:

banner

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'one ruling' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for banner that I've seen before include "Long strip with a slogan" , "Web page header; flag" , "Website advert; flag" , "Ensign" , "Long flag" .)

I've seen this clue in The Times.

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