Usually working, affirm how old one is (2,7)
I believe the answer is:
Here is my best explanation:
'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'.
(Other definitions for on average that I've seen before include "typically speaking", "Usually", "Generally" and "Typically".)