Japanese islanders, initially aboriginal in northern union (4)
I believe the answer is:
ainu
'japanese' is the definition.
(I've seen this before)
'initially aboriginal in northern union' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'aboriginal in northern union' is 'AINU'.
'islanders' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ainu that I've seen before include "language" , "Big Oriental" , "Japanese person" , "aborigines" .)