Is again on the river (4)
I believe the answer is:
isis
'river' is the definition.
(term for the River Thames in Oxford)
'is again on' is the wordplay.
'again' becomes 'is' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'is' after 'is' is 'ISIS'.
'the' is the link.
Can you help me to learn more?
(Other definitions for isis that I've seen before include "Oxford river; goddess" , "Wife of Osiris" , "Oxford's second crew" , "Exists twice for a goddess" , "where one might take a punt" .)