Mineral used as an insulator (4)
I believe the answer is:
mica
'mineral used as an insulator' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mica that I've seen before include "Shiny silicate mineral" , "Isinglass" , "Muscovite with perfect cleavage" , "Flexible mineral found in igneous and metamorphic rocks" , "Semitransparent insulating mineral" .)