Type of rock providing road-building material (4,4)
I believe the answer is:
hard core
I believe this is a double definition.
'type of rock' is the first definition.
(type of rock music)
'road-building material' is the second definition.
(material used for making roads)
'providing' is the link.
(Other definitions for hard core that I've seen before include "Why some apple is uneaten" , "The irreducible minimum" , "Pornography" , "filthy" , "filth?" .)