Knock back drink, last of drink in gloom (4)
I believe the answer is:
murk
'gloom' is the definition.
(I've seen this before)
'knock back drink last of drink' is the wordplay.
'knock back' says the letters should be written in reverse.
'drink' becomes 'rum' (I've seen this before**).
'last of' says to take the final letters.
The final letter of 'drink' is 'k'.
'rum' backwards is 'mur'.
'mur'+'k'='MURK'
'in' acts as a link.
(Other definitions for murk that I've seen before include "Reduced visibility" , "Gloomy darkness" , "Foggy darkness" , "Dark gloom" , "Foul darkness" .)