Complain quietly after knocking back odd drink (6)
I believe the answer is:
murmur
'complain quietly' is the definition.
'murmur' can be an answer for 'complain' (murmuring is a kind of complaining). I am not sure about the 'quietly' bit.
'knocking back odd drink' is the wordplay.
'knocking back' says the letters should be written in reverse.
'odd' becomes 'rum' (rum can mean strange or dubious).
'drink' becomes 'rum' (rum is a kind of drink).
'rum'+'rum'='rumrum'
'rumrum' back-to-front is 'MURMUR'.
'after' is the link.
Can you help me to learn more?
(Other definitions for murmur that I've seen before include "Low continuous sound" , "Low background noise" , "Mumble" , "Soft voices (of discontent)" , "Speak in a low voice, whisper" .)