Speck of soot or dirt (4)
I believe the answer is:
smut
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">speck of soot</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">dirt</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for smut that I've seen before include "Dirt or obscenity" , "Sooty deposit" , "Small soot flake or indecent talk" , "Obscene stuff" , "Soot; obscenities" .)