Gloom emerges as last of dark spirit is knocked back (4)
I believe the answer is:
murk
'gloom' is the definition.
(I've seen this before)
'last of dark spirit is knocked back' is the wordplay.
'last of' indicates one should take the final letters.
'spirit' becomes 'rum' (alcoholic spirit).
'is knocked back' says the letters should be written backwards.
The final letter of 'dark' is 'k'.
'k'+'rum'='krum'
'krum' back-to-front is 'MURK'.
'emerges as' is the link.
(Other definitions for murk that I've seen before include "Darkness, fog" , "Reduced visibility" , "Dark gloom" , "Foggy darkness" , "Gloomy darkness" .)