Knocks back with bent elbow (6)

I believe the answer is:

quaffs

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'knocks back with bent elbow' is the definition.
Although both the answer and definition are verbs in their -s form, I can't understand how they can define each other.

This is all the clue.

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(Other definitions for quaffs that I've seen before include "Drinks rapidly" , "Drinks in long draughts" , "Swigs" , "Drinks in long swallows" , "Drinks heartily" .)

I've seen this clue in the Evening Standard.

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