Crossword Genius

His cat Choppy terribly mentally disturbed (12)

Ross

I believe the answer is:

psychopathic

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'disturbed' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

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(Other definitions for psychopathic that I've seen before include "Pathologically violent and antisocial" , "certifiable" .)

I've seen this clue in the Sydney Morning Herald.
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