Filth, dirt (5)
I believe the answer is:
grime
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">filth</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that filth can be written as grime)</span><br/ ><br/ >'<span class="explanation-format__original">dirt</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(grime can mean dirt or mud)</span><br/ ><br/ ></span>
(Other definitions for grime that I've seen before include "Ingrained dirt" , "Dirt, grease" , "Soot" , "Engrained dirt" , "Muck" .)