Bicuspid coating (6)
I believe the answer is:
enamel
'bicuspid coating' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for enamel that I've seen before include "Hard shiny coating on surface" , "Vitrified coating for metal etc" , "Hard shiny coating as on teeth" , "found on crowns" , "Decorative inlay" .)