It forms on the teeth (6)
I believe the answer is:
tartar
'it forms on the teeth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tartar that I've seen before include "may be hard to brush off" , "Cold sauce, popular with fish" , "Hard, crusty deposit on the teeth" , "Deposit that forms on teeth" , "Crust on teeth or wine deposit" .)