The most common and most likely answer to this crossword clue is the 4 letter word CALX
C
A
L
X
Buddy explains!
The straight part
We know that "Mineral remains" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mineral remains = calx.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.
What does CALX mean?
CALX noun- The substance which remains after a metal or mineral has been thoroughly burnt, once seen as being the essential substance left after the expulsion of phlogiston, but now recognised as being the metallic oxide (or, in some cases, the metal in a state of sublimation).
- In the Eton College wall game, an area at the end of the field where a shy can be scored by lifting the ball against the wall with one's foot.