Tooth covering (6)
I believe the answer is:
enamel
'tooth covering' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for enamel that I've seen before include "Hard glossy surface" , "found on crowns" , "Hard shiny coating as on teeth" , "Crown Protector?" , "Hard white substance covering the crown of a tooth" .)