Excellent beginning of reggae song (3)
I believe the answer is:
air
'song' is the definition.
(air can mean a piece of music)
'excellent beginning of reggae' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'beginning of' indicates taking the first letters.
The initial letter of 'reggae' is 'r'.
'ai'+'r'='AIR'
(Other definitions for air that I've seen before include "Cause of inflation" , "Look - warm and dry" , "A tune or attitude" , "Tune; mien" , "Skye Boat Song, perhaps" .)