The most common and most likely answer to this crossword clue is the 7 letter word APATITE
I've seen this crossword clue in The Guardian
A
P
A
T
I
T
E
Buddy explains!
The straight part
We know that "A mineral" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a mineral = apatite.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 APATITE mean?
APATITE noun- A calcium fluoride phosphate of variable composition, sometimes used in the manufacture of fertilizer.