Flaky mineral (4)
I believe the answer is:
mica
'flaky mineral' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)