Kingdom, United Kingdom, overturned Remain (6)
I believe the answer is:
kuwait
'kingdom' is the definition.
(I know that Kuwait is a kingdom)
'united kingdom overturned remain' is the wordplay.
'united kingdom' becomes 'UK' (synonyms).
'overturned' shows that the letters should be reversed in order.
'remain' becomes 'wait' (wait can mean to remain or stay somewhere).
'uk' reversed gives 'ku'.
'ku'+'wait'='KUWAIT'
(Other definitions for kuwait that I've seen before include "Saddam Hussein invaded this country on Persian Gulf" , "Country on Persian gulf coveted by Saddam Hussein" , "Arab kingdom on the Gulf" , "Arab kingdom, major source of oil" , "Oil-rich country" .)