Relative in Foulness Island (3)
I believe the answer is:
sis
'relative' is the definition.
(short for sister)
'in foulness island' is the wordplay.
'in' indicates a hidden word.
'SIS' can be found hidden inside 'foulness island'.
(Other definitions for sis that I've seen before include "relative" , "Little sibling" , "spies ..." , "Girl" , "Secret Intelligence Service (abbreviation)" .)