Banish in former French island (5)
I believe the answer is:
exile
'banish' is the definition.
(thesaurus)
'former french island' is the wordplay.
'former' becomes 'ex'.
'french island' becomes 'ile' (French for 'island').
'ex'+'ile'='EXILE'
'in' is the link.
(Other definitions for exile that I've seen before include "Eject from country" , "Ostracism" , "eg Napoleon on Elba" , "Ban to abroad" , "Overseas banishment" .)