Dark substance used in road surfaces (7)
I believe the answer is:
asphalt
'dark substance used in road surfaces' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for asphalt that I've seen before include "surface of path" , "Dark natural pitch" , "petroleum residue" , "Bituminous substance used for roofs and roads" , "Paving material" .)