Tobacco residue (3)
I believe the answer is:
tar
'tobacco residue' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tar that I've seen before include "Informal sailor or coal distillation" , "Thick, sticky liquid distilled from coal" , "Product of smoking cigarettes" , "Sticky stuff for a kind of sailor" , "used by those who pay to work on deck" .)