Almost all the booze knocked back in darkness (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a kind of darkness)
'almost all the booze knocked back' is the wordplay.
'almost all' means to remove the last letter.
'booze' becomes 'gin' (gin is a kind of booze).
'knocked back' says the letters should be written backwards.
'the' with its last letter removed is 'th'.
'th'+'gin'='thgin'
'thgin' back-to-front is 'NIGHT'.
'in' is the link.
(Other definitions for night that I've seen before include "Illumination needed now" , "''Twas the ... before Christmas, when all through the house / Not a creature was stirring, not even a mouse''" , "'Red sky at . . . . ., shepherd's delight'" , "Time between days" , "Time for turning" .)