Old family, one at war endlessly somewhere in Japan (7)
I believe the answer is:
okinawa
'somewhere in japan' is the definition.
(part of Japan)
'old family one at war endlessly' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'family' becomes 'kin' (synonyms).
'one' becomes 'a' ().
'at' says to put letters next to each other.
'endlessly' means to remove the last letter.
'war' with its final letter removed is 'wa'.
'o'+'kin'+'a'+'wa'='OKINAWA'
(Other definitions for okinawa that I've seen before include "coral island" , "Campaign in the closing days of World War II in the Pacific" , "Japanese location" , "American victory, June 1945" , "island out east" .)