Relative in this island (3)
I believe the answer is:
sis
'relative' is the definition.
(short for sister)
'in this island' is the wordplay.
'in' indicates a hidden word.
'SIS' is hidden in the letters of 'this island'.
(Other definitions for sis that I've seen before include "Family member (informal)" , "Girl" , "Secret Intelligence Service (abbreviation)" , "Secret Intelligence Service (abbr.)" , "Sibling (informal)" .)