Knocks back with bent elbow (6)
I believe the answer is:
'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.
Can you help me to learn more?
(Other definitions for quaffs that I've seen before include "Drinks rapidly" , "Drinks in long draughts" , "Swigs" , "Drinks in long swallows" , "Drinks heartily" .)