King's one to upset country (6)
I believe the answer is:
israel
'country' is the definition.
(Israel is an example)
'king's one to upset' is the wordplay.
'king's' becomes 'lear's' (King Lear).
'one' becomes 'I' (Roman numeral).
'to upset' says the letters should be written in reverse.
'lears'+'i'='learsi'
'learsi' in reverse letter order is 'ISRAEL'.
(Other definitions for israel that I've seen before include "Jaffa's country" , "Zion" , "Middle Eastern state" , "Eastern Med nation" , "Middle-Eastern state" .)