Two articles on composer's third patriotic song (6)
I believe the answer is:
anthem
'patriotic song' is the definition.
(a national anthem)
'two articles on composer's third' is the wordplay.
'two articles' can mean two replacements for 'article'.
'article' becomes 'an' (English indefinite article).
'article' becomes 'the' (the definite article in English).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'composer's third' becomes 'm' (3rd letter of 'composer').
'an'+'the'+'m'='ANTHEM'
(Other definitions for anthem that I've seen before include "Uplifting song -- the man (anag)" , "air of patriotism?" , "The man for a patriotic song" , "National melody" , "Religious chant" .)