One short tune being broadcast (2,3)
I believe the answer is:
on air
'being broadcast' is the definition.
(I've seen this before)
'one short tune' is the wordplay.
'short' means to remove the last letter.
'tune' becomes 'air' (I've seen this before).
'one' with its final letter removed is 'on'.
'on'+'air'='ON AIR'
(Other definitions for on air that I've seen before include "Transmitting now!" , "Cushioned" , "Broadcasting live" , "In the process of broadcasting - floating aloft?" , "Broadcasting studio sign" .)