Oxford river with twin islands (4)
I believe the answer is:
isis
'oxford river' is the definition.
(term for the Thames used in Oxford)
'with twin islands' is the wordplay.
'twin islands' means two replacements for 'islands'.
'with' is a charade indicator (letters next to each other).
'islands' becomes 'is' (i is an abbreviation for island).
'islands' becomes 'is' (i is an abbreviation for island).
'is'+'is' is 'ISIS'.
(Other definitions for isis that I've seen before include "Wife of Osiris" , "Oxford river; goddess" , "A dark blue flower" , "Egyptian goddess of nature" , "Goddess of the rainbow" .)