Mineral used in ancient alchemy (4)
I believe the answer is:
talc
'mineral' is the definition.
(talc is a kind of mineral)
'in ancient alchemy' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TALC' can be found hidden inside 'ancient alchemy'.
'used' is the link.
(Other definitions for talc that I've seen before include "Toilet powder - mica" , "Mica - scented body powder" , "Softest substance in Mohs scale of hardness" , "Softest mineral on Mohs hardness scale" , "Fine powder" .)