Hard stuff in black and slimy mud? (5)
I believe the answer is:
booze
'hard stuff' is the definition.
(I've seen this in another clue)
'black and slimy mud?' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'and' says to put letters next to each other.
'slimy mud?' becomes 'ooze' (I've seen this in another clue).
'b'+'ooze'='BOOZE'
'in' is the link.
(Other definitions for booze that I've seen before include "Alcohol (sl.)" , "intoxicating stuff" , "sauce" , "Drink heavily" , "In slang, alcoholic drink" .)