Drink up, mate (3)

I believe the answer is:

pal

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This could be a double definition.

'drink up' is the first definition.
Although both the answer and definition are verbs in their base form, I don't understand how they can define each other.

'mate' is the second definition.
(thesaurus)

Can you help me to learn more?

(Other definitions for pal that I've seen before include "Bud" , "Crony" , "Brother" , "Close friend" , "China" .)

I've seen this clue in the Evening Standard.

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