Short single, tune being broadcast (2,3)
I believe the answer is:
on air
'being broadcast' is the definition.
(on air on radio or TV)
'short single tune' is the wordplay.
'short' means to remove the last letter.
'single' becomes 'one' (a single thing - only one).
'tune' becomes 'air' (air can mean a piece of music).
'one' with its final letter taken off is 'on'.
'on'+'air'='ON-AIR'
(Other definitions for on air that I've seen before include "How one walks when elated" , "(Walking) elatedly" , "In the process of broadcasting - floating aloft?" , "Cushioned" , "put out" .)