Deposit that forms on teeth (6)
I believe the answer is:
tartar
'deposit that forms on teeth' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tartar that I've seen before include "Hard, crusty deposit on the teeth" , "Dental deposit" , "Cold sauce, popular with fish" , "saucy stuff" , "Fearsome person: kind of sauce" .)