Playing a specific melody live (2,3,3)
I believe the answer is:
on the air
'live' is the definition.
(I've seen this before)
'playing a specific melody' is the wordplay.
'playing' becomes 'on' (**).
'a specific' becomes 'the'.
'melody' becomes 'air' ('air' can be a synonym of 'melody').
'on'+'the'+'air'='ON THE AIR'
(Other definitions for on the air that I've seen before include "Broadcasting (hence audible)" , "Anti-hero engaged in live broadcast" , "In the process of broadcasting, radio or TV" , "radio listeners can now hear?" .)