Deposit that forms on teeth (6)
I believe the answer is:
tartar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">deposit that forms on teeth</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)