Softest mineral (4)
I believe the answer is:
talc
'softest mineral' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for talc that I've seen before include "After-bath powder" , "Soft mineral; powder" , "Drier" , "Scented powder used after bathing" , "Perfumed drying powder" .)