Usually working, affirm how old one is (2,7)
I believe the answer is:
on average
'usually' is the definition.
(I've seen this before)
'working affirm how old one is' is the wordplay.
'working' becomes 'on'.
'affirm' becomes 'aver' ('aver' can be a synonym of 'affirm').
'how old one is' becomes 'age'.
'on'+'aver'+'age'='ON AVERAGE'
(Other definitions for on average that I've seen before include "Generally" , "typically speaking" , "Usually" .)