Dental calculus (6)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">dental calculus</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tartar that I've seen before include "may be hard to brush off" , "Sauce often accompanying fish" , "Deposit that forms on teeth" , "ancient Mongolian" , "Unpleasant sort" .)