Dark viscid substance (3)
I believe the answer is:
tar
'dark viscid substance' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)