Crossword Genius

Prohibitive placard? (6)

Ross

I believe the answer is:

banner

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This might be a double definition.

'prohibitive' is the first definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?

'placard?' is the second definition.
(a placard is a banner or poster)

Can you help me to learn more?

(Other definitions for banner that I've seen before include "Long flag" , "Knight's standard" , "Standard carried at demo etc" , "Flag used as standard" , "Flag bearing a device" .)

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