Pacific Ocean monarchy (5)
I believe the answer is:
tonga
'pacific ocean monarchy' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tonga that I've seen before include "Pacific island state" , "Two-wheeled Indian vehicle" , "Pacific Ocean monarchy" , "South Pacific kingdom, capital Nuku'alofa" , "part of Polynesia" .)