City on the Douro River (6)
I believe the answer is:
oporto
'city on the douro river' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for oporto that I've seen before include "Portuguese wine city" , "Iberian city" , "European city" , "Wine city in Portugal" , "Second largest city in Portugal" .)