Broadcast, at first, completely commercial song (6)
I believe the answer is:
ballad
'song' is the definition.
(ballad is a kind of song)
'broadcast at first completely commercial' is the wordplay.
'at first' suggests taking the first letters.
'completely' becomes 'all' ('all' can be a synonym of 'completely').
'commercial' becomes 'ad'.
The initial letter of 'broadcast' is 'b'.
'b'+'all'+'ad'='BALLAD'
(Other definitions for ballad that I've seen before include "Folk song" , "Form of verse" , "romance" , "Slow romantic song" , "Slow sentimental song" .)