Sailor also called foreign port (5)
I believe the answer is:
'foreign port' is the definition.
(port in Japan)
'sailor also called' is the wordplay.
'sailor' becomes 'os' (ordinary seaman).
'also called' becomes 'aka' (also known as).
(Other definitions for osaka that I've seen before include "Third most populous Japanese city" , "Kinki's capital" , "Place" , "Japanese metropolis" , "Large city and port in Japan" .)