Filth, dirt (5)
I believe the answer is:
grime
This clue is a double definition.
'filth' is the first definition.
(I know that filth can be written as grime)
'dirt' is the second definition.
(grime can mean dirt or mud)
(Other definitions for grime that I've seen before include "Ingrained dirt" , "Dirt, grease" , "Soot" , "Engrained dirt" , "Muck" .)