Vermilion mineral (8)
I believe the answer is:
cinnabar
'vermilion mineral' is the definition.
(red mineral and historic source of vermillion)
This is all the clue.
(Other definitions for cinnabar that I've seen before include "Heavy red mineral containing mercury" , "Red mercury sulphide" , "Brian can make vermilion pigment" , "Bright red mineral containing mercury" .)