Knocked back red with rum; it’s killing! (6)
I believe the answer is:
murder
'it's killing' is the definition.
(murder is a kind of killing)
'knocked back red with rum' is the wordplay.
'knocked back' says the letters should be written in reverse.
'with' says to put letters next to each other.
'red'+'rum'='redrum'
'redrum' reversed gives 'MURDER'.
(Other definitions for murder that I've seen before include "Red Rum (anag)" , "Dispatch" , "Deliberate killing" , "Killing most foul" , "Shoot dead" .)