Am to break, then, into song (6)
I believe the answer is:
anthem
'song' is the definition.
(anthem is a kind of song)
'am to break then into' is the wordplay.
'to break' indicates anagramming the letters.
'into' indicates putting letters inside.
'then' with letters rearranged gives 'nthe'.
'am' enclosing 'nthe' is 'ANTHEM'.
(Other definitions for anthem that I've seen before include "Choral work" , "One for doomed youth" , "The man for a popular song" , "Religious chant" , "National tune" .)