Mucky mix (6)
I believe the answer is:
sludge
'mucky mix' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sludge that I've seen before include "Thick greasy mud" , "Melting snow" , "Gloop, gunk, muck" , "Soft mud; slimy waste" , "Mire - sewage" .)