Japanese port — old seaport Asians know about initially (5)
I believe the answer is:
osaka
'japanese port' is the definition.
(I've seen this before)
'old seaport asians know about initially' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'old seaport asians know about' is 'OSAKA'.
(Other definitions for osaka that I've seen before include "Large city and port in Japan" , "in east Asia" , "Japanese metropolis" , "Port city of southern Honshu" , "Japan's third-largest city" .)