Performing well but not off air (2,4)
I believe the answer is:
on song
'performing well' is the definition.
(I've seen this before)
'not off air' is the wordplay.
'not off' becomes 'on' ('on' is the opposite to 'off').
'air' becomes 'song' (I've seen this before**).
'on'+'song'='ON SONG'
'but' is the link.
(Other definitions for on song that I've seen before include "Vocally" , "Performing well (informal)" , "Performing well - in form (slang)" , "In superlative form" , "in fine form" .)