Ducks seen outside harbour town and city (6)
I believe the answer is:
oporto
'city' is the definition.
(I've seen this before)
'ducks seen outside harbour' is the wordplay.
'ducks' becomes 'oo' (I've seen this in other clues).
'seen outside' means one lot of letters goes inside another.
'harbour' becomes 'port' (I've seen this before).
'oo' enclosing 'port' is 'OPORTO'.
'town and' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for oporto that I've seen before include "foreign place" , "Portuguese wine trade centre" , "Iberian location" , "Wine city in Portugal" , "Iberian city" .)