Softest of minerals (4)
I believe the answer is:
talc
'softest of minerals' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for talc that I've seen before include "Softest mineral on Mohs hardness scale" , "(Mineral providing) powder" , "After-bath powder" , "Scented powder used after bathing" , "Softest substance in Mohs scale of hardness" .)