Dirt, filth (4)
I believe the answer is:
muck
This clue is a double definition.
'dirt' is the first definition.
(I've seen this in another clue)
'filth' is the second definition.
(I've seen this in another clue)
(Other definitions for muck that I've seen before include "Dirt, manure" , "Trash" , "Dirt, mud" , "brass accompaniment?" , "Filth (where there's brass?)" .)