Crossword Genius

What might be read as warning to the mob? (4,3)

Ross

I believe the answer is:

riot act

'mob?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

I can't explain the remainder of the clue.

(Other definitions for riot act that I've seen before include "Citator may read this as stern rebuke" , "something read to little devils?" , "what constitutes warning" , "Citator of law concerning public affray" , "designed to nip trouble in the bud" .)

I've seen this clue in The Times.
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