Deposit on the teeth (6)
I believe the answer is:
tartar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">deposit on the 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 "Dental deposit" , "Unpleasant sort" , "For instance, Genghis Khan" , "Hard, crusty deposit on the teeth" , "saucy stuff" .)