River one's to cross outside of Shanghai (4)
I believe the answer is:
isis
'river' is the definition.
(term for the River Thames in Oxford)
'one's to cross outside of shanghai' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'cross' indicates anagramming the letters (a cross or irritated version of the letters).
'outside of' means to remove the middle letters.
'shanghai' with its middle removed is 'si'.
'si' with letters rearranged gives 'is'.
'is' put after 'is' is 'ISIS'.
(Other definitions for isis that I've seen before include "where one might take a punt" , "Oxfordshire river" , "Repetitive Egyptian goddess" , "banker in Oxford" , "Si Si - she's an Egyption goddess" .)