Old and new tune being broadcast (2,3)
I believe the answer is:
on air
'being broadcast' is the definition.
(on air on radio or TV)
'old and new tune' is the wordplay.
'old' becomes 'o' (abbreviation).
'and' means one lot of letters go next to another.
'new' becomes 'n' (abbreviation as in New Testament NT).
'tune' becomes 'air' (air can mean a piece of music).
'o'+'n'+'air'='ON-AIR'
(Other definitions for on air that I've seen before include "Broadcasting live" , "In the act of broadcasting" , "Broadcasting at this moment" , "now showing" , "Broadcasting studio sign" .)