Soft white or grey mineral (6)
I believe the answer is:
gypsum
'soft white or grey mineral' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gypsum that I've seen before include "Mineral used in making plaster of Paris" , "Alabaster" , "Ingredient of fertilizer" , "Ingredient of plaster of Paris" , "Mineral used to make plaster of Paris" .)