Family holidaying mostly on old Pacific island (7)
I believe the answer is:
okinawa
'pacific island' is the definition.
(I've seen this before)
'family holidaying mostly on old' is the wordplay.
'family' becomes 'kin' (synonyms).
'holidaying mostly' becomes 'awa' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' is a charade indicator (letters next to each other).
'old' becomes 'o' (abbreviation).
'kin'+'awa'='kinawa'
'kinawa' put after 'o' is 'OKINAWA'.
Can you help me to learn more?
(Other definitions for okinawa that I've seen before include "Campaign in the closing days of World War II in the Pacific" , "American victory, June 1945" , "Island of S. Japan" , "Pacific island" , "battle!" .)