Fool I had taken back to island (5)
I believe the answer is:
idiot
'fool' is the definition.
(thesaurus)
'i had taken back to island' is the wordplay.
'i had' becomes 'id' (I'd).
'taken back' says the letters should be written in reverse.
'island' becomes 'i'.
'to'+'i'='toi'
'toi' reversed gives 'iot'.
'id'+'iot'='IDIOT'
(Other definitions for idiot that I've seen before include "Village character" , "Simpleton" , "Ass, ninny" , "Garret" , "Foolish person" .)