Crossword Genius

Man being one to deceive not outwardly bonkers? (4)

Ross

I believe the answer is:

isle

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

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for isle that I've seen before include "-- of Dogs, area of London" , "Sea-girt area" , "-- of Ely, Dogs, Man" , "Detached territory" , "Piece of land in water" .)

I've seen this clue in The Times.
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