Alert? No, one may surmise? (8,2)

I believe the answer is:

switched on

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'alert?' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for switched on that I've seen before include "Illuminated" , "Alert" , "Up to date" , "Aware" , "Now listening to Radio 4, say" .)

I've seen this clue in The Times.

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