It's 1 on the Mohs scale (4)
I believe the answer is:
talc
'it's 1 on the mohs scale' 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 "Soft stuff" , "White powder for the bathroom" , "Soft white powder" , "Bathroom dusting powder" , "Magnesium silicate powder" .)