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