Rejecting drinking (8,4)

I believe the answer is:

knocking back

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This clue might be a double definition.

'rejecting' is the first definition.
Although both the answer and definition are present participle verbs, I don't understand how they can define each other.

'drinking' is the second definition.
Although both the answer and definition are present participle verbs, I don't see how they can define each other.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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