Île de la Cité surrounder (5)
I believe the answer is:
seine
'ile de la cite surrounder' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for seine that I've seen before include "French river -- type of fishing net" , "Paris river" , "Parisian river" , "Paris's river" , "Second longest river in France" .)