A drink of sap? (3)

I believe the answer is:

mug

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'sap?' is the definition.
(thesaurus)

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

(Other definitions for mug that I've seen before include "Drinking vessel; dupe" , "Gullible type; drinking vessel" , "Face; cup; rob" , "Drinking vessel, by gum" , "Cup; stupid person" .)

I've seen this clue in the Evening Standard.

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