Speck of soot (4)
I believe the answer is:
smut
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">speck of soot</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for smut that I've seen before include "Cereal disease; filth" , "Dirt or immoral talk" , "Dirty stuff" , "Dirty jokes - black mark" , "Dirt or pornography" .)