A mug of vegetable juice (3)

I believe the answer is:

sap

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This might be a double definition.

'a 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 certain of the 'vegetable' bit.

'of' is the link.

Can you help me to learn more?

(Other definitions for sap that I've seen before include "Enervate" , "Plant juice for silly ass" , "Undermine; fool" , "Moisture in wood of a tree" , "Watery substance in a plant" .)

I've seen this clue in the Evening Standard.

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