Mineral used for plaster of Paris (6)
I believe the answer is:
gypsum
'mineral used for plaster of paris' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gypsum that I've seen before include "Spy mug made from this white mineral" , "Base of plaster of Paris" , "Plaster of Paris is made from this chalk-like substance" , "Ingredient of plaster of Paris" , "White mineral in cement and plaster" .)