Plaster of Paris mineral (6)
I believe the answer is:
gypsum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">plaster of paris mineral</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)