Quick signal (5)

I believe the answer is:

alert

I believe this clue is a double definition.

'quick' is the first definition.
(I've seen this before)

'signal' is the second definition.
(I know that alert is a type of signal)

(Other definitions for alert that I've seen before include "On the ball" , "Watchful and bright" , "Lively" , "In state of watchfulness" , "Watchful, quick-thinking" .)

I've seen this clue in The Guardian.

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