Excellent reason to start with song (3)
I believe the answer is:
air
'song' is the definition.
(I've seen this before)
'excellent reason to start' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'to start' suggests taking the first letters.
The first letter of 'reason' is 'r'.
'ai'+'r'='AIR'
'with' acts as a link.
(Other definitions for air that I've seen before include "Demeanour" , "Song - atmosphere" , "Broadcast; tune" , "Tune (on the G string?)" , "Space above and around" .)