An absorbing and refreshing thing to do (3)

I believe the answer is:

sup

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'do' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I can't explain the rest of the clue.

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(Other definitions for sup that I've seen before include "Drink; eat evening meal" , "A mouthful of liquid" , "Take food" , "Eat a late meal; sip" , "Drink -- eat (with the devil?)" .)

I've seen this clue in the Evening Standard.

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