National football team containing king's back (7)
I believe the answer is:
israeli
'national' is the definition.
(someone from Israel)
'football team containing king's back' is the wordplay.
'football team' becomes 'II' (resembles ll, a football team).
'containing' means one lot of letters goes inside another.
'king's' becomes 'lears' (King Lear).
'back' says the letters should be written backwards.
'ii' going around 'lears' is 'ilearsi'.
'ilearsi' backwards is 'ISRAELI'.
(Other definitions for israeli that I've seen before include "of the Levant?" , "Statesman" , "SW Asian" , "Nationality of citizen of Tel Aviv" , "from part of Middle East?" .)