There's nothing left, love, in the city (6)
I believe the answer is:
'city' is the definition.
(city in Portugal)
'there's nothing left love' is the wordplay.
'there's nothing' becomes 'o' (I've seen this in other clues).
'left' becomes 'port' (left side of a ship - opposite of starboard).
'love' becomes 'o' (love means zero in tennis).
'in the' is the link.
(Other definitions for oporto that I've seen before include "City near the mouth of the River Douro" , "Iberian city" , "Iberian location" , "Portuguese wine trade centre" , "Wine city in Portugal" .)