Commotion starts to signal trouble in restaurant (4)

I believe the answer is:

stir

'commotion' is the definition.
('stir' can be a synonym of 'commotion')

'starts to signal trouble in restaurant' is the wordplay.
'starts to' says to take the initial letters.
The first letters of 'signal trouble in restaurant' is 'STIR'.

(Other definitions for stir that I've seen before include "Slight movement" , "Whisk, blend" , "Ado" , "Arouse; commotion" , "Whisk or make a move" .)

I've seen this clue in The Telegraph.

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