Deposit on teeth (6)
'deposit on teeth' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tartar that I've seen before include "Deposit that forms on teeth" , "Incrustation on teeth and gums" , "ancient Mongolian" , "A sauce to go with fish" , "For instance, Genghis Khan" .)
(Other definitions for plaque that I've seen before include "Substance that forms on teeth encouraging bacteria" , "dental hygiene problem" , "Film on teeth that encourages caries" , "Memorial slab" , "Dental coating" .)