Short one, tune being broadcast (2,3)
I believe the answer is:
on air
'being broadcast' is the definition.
(on air on radio or TV)
'short one tune' is the wordplay.
'short' means to remove the last letter.
'tune' becomes 'air' (air can mean a piece of music).
'one' with its final letter taken away is 'on'.
'on'+'air'='ON-AIR'
(Other definitions for on air that I've seen before include "Cushioned" , "Live broadcast" , "Broadcasting studio sign" , "Currently broadcasting" , "Broadcasting at this moment" .)