Knock back drink on bar (12)
I believe the answer is:
counterpunch
'knock' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'back drink on bar' is the wordplay.
'back drink' becomes 'punch' (punch is a kind of drink. I am not sure about the 'back' bit.**).
'on' says to put letters next to each other.
'bar' becomes 'counter' (I've seen this before).
'punch' put after 'counter' is 'COUNTERPUNCH'.
(Another definition for counterpunch that I've seen is " Return blow in boxing".)