Generally no different to mean (2,7)
I believe the answer is:
on average
'generally' is the definition.
(I've seen this before)
'no different to mean' is the wordplay.
'different' is an anagram indicator.
'to mean' becomes 'average' (synonyms).
'no' with letters rearranged gives 'on'.
'on'+'average'='ON AVERAGE'
(Other definitions for on average that I've seen before include "Usually" , "typically speaking" , "Generally" .)