King backed by two-thirds of his country (6)
I believe the answer is:
israel
'country' is the definition.
(I know that Israel is a country)
'king backed by two-thirds of his' is the wordplay.
'king' becomes 'lear' (Shakespeare's King Lear).
'backed' says the letters should be written backwards.
'by' says to put letters next to each other.
'two-thirds of his' becomes 'is' (I can't explain this - if you can you should believe this answer much more).
'lear' backwards is 'rael'.
'rael' put after 'is' is 'ISRAEL'.
Can you help me to learn more?
(Other definitions for israel that I've seen before include "Eastern Med nation" , "Middle Eastern state" , "Ale, sir, can be found in the country" , "Mediterranean republic" , "Serial about Jewish nation" .)