Man being one to deceive not outwardly bonkers? (4)
I believe the answer is:
isle
'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" .)