Flaky mineral (4)
I believe the answer is:
mica
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">flaky mineral</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for mica that I've seen before include "Flexible mineral found in igneous and metamorphic rocks" , "Muscovite with perfect cleavage" , "Semitransparent insulating mineral" , "Shiny silicate mineral" , "Transparent mineral" .)