Shiny mineral (4)
I believe the answer is:
mica
'shiny 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" , "Transparent mineral" , "Semitransparent insulating mineral" , "Shiny silicate mineral" , "Isinglass" .)