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