Site of the only royal palace in the US (4)
I believe the answer is:
oahu
'site of the only royal palace in the us' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oahu that I've seen before include "Pacific island" , "Chief island of Hawaii" , "Hawaiian island" , "Honolulu's island" , "popular Pacific location" .)