A mug of juice? (3)

I believe the answer is:

sap

This clue is a double definition.

'a mug' is the first definition.
(thesaurus)

'juice?' is the second definition.
(juice from a plant)

'of' acts as a link.

(Other definitions for sap that I've seen before include "Vital plant juice" , "Internal tree moisture" , "Insidiously undermine" , "The juice in trees" , "Juice; energy" .)

I've seen this clue in the Evening Standard.

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