Powder mineral (4)
I believe the answer is:
talc
'powder 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 "Bathroom powder (abbr)" , "Baby powder" , "After-bath powder" , "(Mineral providing) powder" , "Softest mineral on Mohs hardness scale" .)