Mineral unknown, right in mine — end of mine (6)
I believe the answer is:
pyrite
'mineral' is the definition.
(pyrite is a kind of mineral)
'unknown right in mine end of mine' is the wordplay.
'unknown' becomes 'y' (algebra).
'right' becomes 'r' (common abbreviation).
'in' indicates putting letters inside.
'mine' becomes 'pit' (down the pit).
'end of' indicates one should take the final letters.
The final letter of 'mine' is 'e'.
'y'+'r'='yr'
'yr' inserted into 'pit' is 'pyrit'.
'pyrit'+'e'='PYRITE'
(Other definitions for pyrite that I've seen before include "Shiny yellow mineral" , "Yellow mineral known as fool's gold" .)