Goddess, one on South Island (4)
I believe the answer is:
isis
'goddess' is the definition.
(thesaurus)
'one on south island' is the wordplay.
'one' becomes 'i' (Roman numeral).
'on' means one lot of letters go next to another.
'south' becomes 's' ('S' can be a synonym of 'south').
'island' becomes 'is'.
'i'+'s'+'is'='ISIS'
(Other definitions for isis that I've seen before include "divinity" , "The goddess lives twice" , "banker in Oxford" , "River Thames at Oxford" , "God" .)