Easily-split mineral
I believe the answer is:
mica
'easily-split 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 "Muscovite with perfect cleavage" , "Semitransparent insulating mineral" , "Shiny silicate mineral" , "Transparent mineral" , "Isinglass" .)