National football team maintaining king's backing (7)
I believe the answer is:
israeli
'national' is the definition.
(someone from Israel)
'football team maintaining king's backing' is the wordplay.
'football team' becomes 'ii' (resembles ll, a football team).
'maintaining' indicates putting letters inside.
'king's' becomes 'lears' (King Lear).
'backing' shows that the letters should be reversed in order.
'ii' enclosing 'lears' is 'ilearsi'.
'ilearsi' back-to-front is 'ISRAELI'.
(Other definitions for israeli that I've seen before include "Man from Tel Aviv" , "Eli's air (anag)" , "countryman" , "from part of Middle East?" , "Jew, possibly" .)