Flue residue; obscene material (4)
I believe the answer is:
smut
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">flue residue</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">obscene material</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for smut that I've seen before include "(Speck) of dirt" , "Piece of dirt or salacious talk" , "blue material" , "Pornography or grime" , "Dirt or immoral talk" .)