Broadcasting, performing tune (2,3)
I believe the answer is:
on air
'broadcasting' is the definition.
(broadcasting on the air)
'performing tune' is the wordplay.
'performing' becomes 'on' (as in being on stage).
'tune' becomes 'air' (air can mean a piece of music).
'on'+'air'='ON-AIR'
(Other definitions for on air that I've seen before include "Live broadcast" , "In the process of broadcasting - floating aloft?" , "Radio show might be" , "Broadcasting at this moment" , "In the act of broadcasting" .)