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 substitutions for 'islands'.
'with' means one lot of letters go next to another.
'islands' becomes 'is' (i is an abbreviation for island).
'islands' becomes 'is' (i is an abbreviation for island).
'is' put next to '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" .)