Performing song live (2,3)
I believe the answer is:
on air
'live' is the definition.
(as in broadcasting live on air)
'performing song' is the wordplay.
'performing' becomes 'on' (as in being on stage).
'song' becomes 'air' (I've seen this before).
'on'+'air'='ON AIR'
(Other definitions for on air that I've seen before include "Live broadcast" , "How one walks when elated" , "(Walking) elatedly" , "In the act of broadcasting" , "Broadcasting at this moment" .)