Covering on teeth (6)
I believe the answer is:
enamel
'covering on teeth' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for enamel that I've seen before include "Coating for a tooth" , "Shiny layer on surface" , "Hard shiny coating as on teeth" , "Hard glossy paint" , "Tooth surface" .)