Crossword Genius

Type of rock providing road-building material (4,4)

Ross

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?" .)

I've seen this clue in The Independent.
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