Glassy coating (6)
I believe the answer is:
enamel
'glassy coating' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for enamel that I've seen before include "Hard glossy surface" , "Tooth surface" , "Decorative or protective covering on metal or glass." , "hard stuff" , "Ale men (anag.)" .)