Japanese port’s extremely obvious alias (5)
I believe the answer is:
osaka
'japanese port's' is the definition.
(I've seen this before)
'extremely obvious alias' is the wordplay.
'extremely' means to remove the middle letters.
'alias' becomes 'aka' ('a.k.a.' can be a synonym of 'alias').
'obvious' with its middle removed is 'os'.
'os'+'aka'='OSAKA'
(Other definitions for osaka that I've seen before include "Port city of southern Honshu" , "S. Japan port, bay" , "Japanese metropolis" , "Japanese port, on Honshu" , "Japanese port, Bay" .)