Broadcasting about the melody (2,3,3)
I believe the answer is:
on the air
'broadcasting' is the definition.
(I've seen this before)
'about the melody' is the wordplay.
'about' becomes 'on' (as in a book 'on' a certain topic).
'melody' becomes 'air' (air can mean a tune or melody).
'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" .)