Drink will make you less than alert (3)

I believe the answer is:

ale

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'drink' is the definition.
(type of beer)

I can't explain the remainder of the clue.

(Other definitions for ale that I've seen before include "brew" , "The A in CAMRA" , "brewing?" , "Beer procured in sales" , "Alcohol" .)

I've seen this clue in the Evening Standard.

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