Crossword Genius

City understanding what the mob might hear? (4,3)

Ross

I believe the answer is:

riot act

I'm a little stuck... Click here to teach me more about this clue!

'what the mob might hear?' is the definition.
I can't tell whether this defines the answer.

'city understanding' is the wordplay.
'city' becomes 'rio' (Rio de Janeiro).
'understanding' becomes 'tact' (I've seen this before).
'rio'+'tact'='RIOT ACT'

Can you help me to learn more?

(Other definitions for riot act that I've seen before include "Citator may read this as stern rebuke" , "used to control disorder" , "measure to keep order" , "what constitutes warning" , "something read to little devils?" .)

I've seen this clue in The Times.
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