Glassy coating (6)
I believe the answer is:
enamel
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">glassy coating</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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.)" .)