Wide awake to a siren call (5)

I believe the answer is:

alert

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'wide awake' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

(Other definitions for alert that I've seen before include "Intellectually active" , "Wakeful, observant" , "Watchful and observant" , "In state of watchfulness" , "On the ball" .)

I've seen this clue in The Guardian and The Globe and Mail.

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