Tar (7)
I believe the answer is:
bitumen
'tar' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bitumen that I've seen before include "coverage on the streets?" , "Road's topping" , "Material for road surfacing, roofing, etc" , "Tarry substance used to surface roads and roofs" , "Mixed-up tar" .)