Crossword Genius

Beside himself, Jack's a fierce fellow (6)

Ross

I believe the answer is:

tartar

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But I can't explain it at all! Could you help me understand?

(Other definitions for tartar that I've seen before include "saucy stuff" , "Crust on teeth or wine deposit" , "may be hard to brush off" , "Unmanageable sort" , "plaque" .)

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