Crossword Genius

One crazy person on French island no good for anything (7)

Ross

I believe the answer is:

inutile

'no good for anything' is the definition.
(inutile means pointless or useless)

'one crazy person on french island' is the wordplay.
'one' becomes 'I' (Roman numeral).
'crazy person' becomes 'nut' (nut can informally mean an insane person).
'on' means one lot of letters go next to another.
'french island' becomes 'ile' ('island' in French).
'i'+'nut'+'ile'='INUTILE'

(Another definition for inutile that I've seen is " Not worth using, useless".)

I've seen this clue in The Telegraph.
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