Mean, performing first, generally (2,7)
I believe the answer is:
on average
'generally' is the definition.
(I've seen this before)
'mean performing first' is the wordplay.
'mean' becomes 'average' (synonyms).
'performing' becomes 'on' (as in being on stage).
'first' says to put letters next to each other.
'average' put after 'on' is 'ON AVERAGE'.
(Other definitions for on average that I've seen before include "Usually" , "Generally" , "typically speaking" .)