State song about endless region (7)
I believe the answer is:
arizona
'state' is the definition.
(Arizona is a US state)
'song about endless region' is the wordplay.
'song' becomes 'aria' (piece of music in an opera).
'about' is an insertion indicator.
'endless' means to remove the last letter.
'region' becomes 'zone' (zone is a kind of region).
'zone' with its last letter taken away is 'zon'.
'aria' enclosing 'zon' is 'ARIZONA'.
(Other definitions for arizona that I've seen before include "State represented by Senators Barry Goldwater and John McCain" , "State of SW USA" , "American state, capital Phoenix" , "part of US" , "American state with Grand Canyon" .)