Bird book is on island (4)
I believe the answer is:
ibis
'bird' is the definition.
(ibis is a kind of bird)
'book is on island' is the wordplay.
'book' becomes 'b'.
'on' is a charade indicator (letters next to each other).
'island' becomes 'i'.
'b'+'is'='bis'
'bis' put after 'i' is 'IBIS'.
(Other definitions for ibis that I've seen before include "Bird, sacred to ancient Egyptians" , "Wader with a long down-curved bill" , "Retreat" , "Large wading birds with down-curved bills" , "Hotel group" .)