Mug of vegetable juice? (3)

I believe the answer is:

sap

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This clue could be a double definition.

'mug' is the first definition.
(thesaurus)

'vegetable juice?' is the second definition.
'sap' can be an answer for 'juice?' (juice from a plant). I am not sure about the 'vegetable' bit.

'of' acts as a link.

Can you help me to learn more?

(Other definitions for sap that I've seen before include "Internal tree moisture" , "Gradually weaken (a person's strength)" , "trench used in siege" , "Weaken; tree fluid" , "Imbecile" .)

I've seen this clue in the Evening Standard.

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