Tablet that isn't good for teeth (6)

I believe the answer is:

plaque

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'tablet' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for plaque that I've seen before include "Film on teeth that encourages caries" , "Sticky deposit on teeth" , "Commemorative tablet - unwanted film on teeth" , "Unhygienic substance that forms on teeth" , "Thin film on teeth" .)

I've seen this clue in The Irish Independent.

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