Part of Oslo, also known as a Japanese port (5)
I believe the answer is:
osaka
'a japanese port' is the definition.
(I've seen this before)
'part of oslo also known as' is the wordplay.
'part of oslo' becomes 'os' (I can't explain this - if you can you should give a lot more credence to this answer).
'also known as' becomes 'aka' (synonyms).
'os'+'aka'='OSAKA'
Can you help me to learn more?
(Other definitions for osaka that I've seen before include "Japan's third-largest city" , "Japan's third city" , "Asian" , "Japanese port, Bay" , "Third largest city in Japan" .)