Soft white mineral (6)
I believe the answer is:
gypsum
'soft white mineral' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gypsum that I've seen before include "Chalky mineral from which plaster of Paris is made" , "Mineral used in making plaster of Paris" , "that provides material for casts" , "Mineral giving plaster of Paris" , "Alabaster" .)