Commotion, having drink knocked back after drink (6)
I believe the answer is:
rumpus
'commotion' is the definition.
(I know that commotion can be written as rumpus)
'drink knocked back after drink' is the wordplay.
'drink' becomes 'rum' (rum is a kind of drink).
'knocked back' says the letters should be written backwards.
'after drink' becomes 'sup' (to sup is to drink. I am not sure about the 'after' bit.).
'sup' backwards is 'pus'.
'rum'+'pus'='RUMPUS'
'having' is the link.
Can you help me to learn more?
(Other definitions for rumpus that I've seen before include "5a [BALLYHOO] involving noise" , "Harsh rough nature" , "Uproar or angry dispute" , "Tumult" , "'Commotion, din (6)'" .)