Drink gallons where plastered member might be? (3,5)

I believe the answer is:

gin sling

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'drink' is the definition.
(gin sling is a kind of drink)

I can't explain the rest of the clue.

(Other definitions for gin sling that I've seen before include "alcoholic drink" , "Spirit flavoured with juniper berries to which lemon or lime juice is added" , "Iced spirit and water sweetened with lemon or lime juice" .)

I've seen this clue in The Sun.

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