Girl in Sinclair Island (4)
I believe the answer is:
iris
'girl' is the definition.
(girl's name)
'in sinclair island' is the wordplay.
'in' indicates a hidden word.
'IRIS' can be found hidden inside 'sinclair island'.
(Other definitions for iris that I've seen before include "Flag or flower in the eye" , "Fleur-de-lis" , "Eye diaphragm" , "Flag - flower - girl" , "--- Murdoch, author" .)