Crossword Genius

A bit off dishonest (6)

Ross

I believe the answer is:

rotten

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'dishonest' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

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(Other definitions for rotten that I've seen before include "Nasty" , "unpleasant" , "Decayed or despicable" , "Informally, very much" , "Lousy" .)

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