Dental calculus
I believe the answer is:
tartar
'dental calculus' 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 "may be hard to brush off" , "Sauce often accompanying fish" , "Deposit that forms on teeth" , "ancient Mongolian" , "Unpleasant sort" .)