Double short knocked back creates complaint (6)
I believe the answer is:
murmur
'complaint' is the definition.
(murmur is a kind of complaint)
'double short knocked back' is the wordplay.
'double' shows that letters should be duplicated.
'short' becomes 'rum' (short can mean an alcoholic drink).
'knocked back' says the letters should be written backwards.
'rum' backwards is 'mur'.
'mur' duplicated is 'MURMUR'.
'creates' is the link.
(Other definitions for murmur that I've seen before include "Subdued complaint" , "Low, indistinct speech" , "Little noise" , "Soft voices (of discontent)" , "Abnormal sound made by heart" .)