Turnpike topper (7)
I believe the answer is:
asphalt
'turnpike topper' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for asphalt that I've seen before include "Bituminous road dressing" , "Paving material" , "Material for road surfacing etc" , "Substance used in roofing and paving" , "Road-buiding material" .)