Crossword Genius

Demonstrating control? (4,3)

Ross

I believe the answer is:

tear gas

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'control?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for tear gas that I've seen before include "Blinding vapour used to subdue" , "Riot control agent" , "Riot police may use this eye irritant" , "Police weapon" , "Riot control substance" .)

I've seen this clue in The Sun.
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