Number upset over the song being broadcast (2,3,3)
I believe the answer is:
on the air
'being broadcast' is the definition.
(I've seen this before)
'number upset over the song' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'upset' is a reversal indicator.
'over' means one lot of letters go next to another.
'song' becomes 'air' (I've seen this before**).
'no' back-to-front is 'on'.
'on'+'the'+'air'='ON THE AIR'
(Other definitions for on the air that I've seen before include "radio listeners can now hear?" , "Anti-hero engaged in live broadcast" , "Broadcasting (hence audible)" , "In the process of broadcasting, radio or TV" .)