Isinglass (4)
I believe the answer is:
mica
'isinglass' is the definition.
(synonyms)
This is all the clue.
(Other definitions for mica that I've seen before include "Transparent mineral" , "Muscovite with perfect cleavage" , "Shiny silicate mineral" , "Semitransparent insulating mineral" , "Isinglass" .)