Black slime in drink (5)
I believe the answer is:
booze
'drink' is the definition.
('booze' can be a synonym of 'drink')
'black slime' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'slime' becomes 'ooze' ('ooze' can be a synonym of 'slime').
'b'+'ooze'='BOOZE'
'in' acts as a link.
(Other definitions for booze that I've seen before include "Drink to excess (colloq.)" , "Alcohol (sl.)" , "Intoxicating liquor" , "Drink too much (colloq.)" , "Drink alcohol" .)