Transmitting old northern song (2,3)
I believe the answer is:
on air
'transmitting' is the definition.
(eg transmitting on the radio)
'old northern song' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'song' 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 "In the process of broadcasting - floating aloft?" , "Radio show might be" , "now showing" , "Walk in delight here" , "Broadcasting studio sign" .)