Drink, black, to leak out slowly (5)
I believe the answer is:
booze
'drink' is the definition.
(booze is a kind of drink)
'black to leak out slowly' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'to leak out slowly' becomes 'ooze' (I've seen this before).
'b'+'ooze'='BOOZE'
(Other definitions for booze that I've seen before include "Alcoholic drink generally" , "Alcoholic liquor" , "sauce" , "intoxicating stuff" , "plonk?" .)