Dark viscid substance (3)
I believe the answer is:
tar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dark viscid substance</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 tar that I've seen before include "Black liquid from coal" , "Sticky stuff for a kind of sailor" , "Informal sailor or road-surfacing material" , "Viscous mixture - sailor" , "Tobacco residue; sailor" .)