The man composing a song (6)
I believe the answer is:
anthem
'a song' is the definition.
(anthem is a kind of song)
'the man composing' is the wordplay.
'composing' indicates anagramming the letters.
'the'+'man'='theman'
'theman' is an anagram of 'ANTHEM'.
(Other definitions for anthem that I've seen before include "National melody" , "National hymn" , "Song of praise or loyalty" , "National tune" , "The man for a rousing song" .)