Usually an over-age disorder (2,7)
I believe the answer is:
on average
'usually' is the definition.
(I've seen this before)
'an over-age disorder' is the wordplay.
'disorder' is an anagram indicator (a disordered version of the letters).
'an'+'over'+'age'='anoverage'
'anoverage' anagrammed gives 'ON AVERAGE'.
(Other definitions for on average that I've seen before include "Generally" , "typically speaking" , "Usually" .)