Great assembly, like to make people disperse with this (4,3)

I believe the answer is:

tear gas

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But I can't explain it at all! Could you help me understand?

(Other definitions for tear gas that I've seen before include "Volatile substance" , "Cause of temporary blindness" , "Riot control substance" , "Riot control agent" , "Crowd disperser" .)

I've seen this clue in The Hindu.

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