The man about to write a song (6)
I believe the answer is:
anthem
'song' is the definition.
(anthem is a kind of song)
'the man about' is the wordplay.
'about' indicates anagramming the letters.
'the'+'man'='theman'
'theman' anagrammed gives 'ANTHEM'.
'to write a' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for anthem that I've seen before include "National or church song" , "Song in praise of a nation" , "Uplifting song -- the man (anag)" , "National hymn" , "National tune" .)