The most common and most likely answer to this crossword clue is the 4 plus 4 letter word HARD CORE
I've seen this crossword clue in Independent
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More clues leading to the result HARD CORE
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The straight part
We know that "Type of rock/road-building material" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case type of rock/road-building material = hard core.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
HARD CORE adjective- Having an extreme dedication to a certain activity; diehard.
- Particularly intense; thrillingly dangerous or erratic; desirably violent in appearance; pleasing or "cool" due to intensity or danger.
- Resistant to change.
- Obscene or explicit.
- Depicting penetration and abnormal sexual activity.
- Faster or more intense than the regular style.