Oxford river with twin islands (4)
I believe the answer is:
isis
'oxford river' is the definition.
(I've seen this before)
'with twin islands' is the wordplay.
'twin islands' means two replacements 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'+'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" .)