A television song with nations competing (Eurovision) starts in disagreement (2,8)
I believe the answer is:
at variance
'in disagreement' is the definition.
(thesaurus)
'a television song with nations competing eurovision starts' is the wordplay.
'television' becomes 'tv' (abbreviation).
'song' becomes 'aria' (piece of music in an opera).
'with' means one lot of letters go next to another.
'starts' suggests taking the first letters.
The first letters of 'nations competing eurovision' is 'nce'.
'a'+'tv'+'aria'+'nce'='AT VARIANCE'
(Other definitions for at variance that I've seen before include "In disagreement or dissension" , "not corresponding" .)