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