Difference isn't changing, in manner of speaking (11)
I believe the answer is:
distinction
'difference' is the definition.
(distinction is a kind of difference)
'isn't changing in manner of speaking' is the wordplay.
'changing' is an anagram indicator.
'in' indicates putting letters inside.
'manner of speaking' becomes 'diction' (I've seen this before).
'isnt' is an anagram of 'stin'.
'stin' inserted into 'diction' is 'DISTINCTION'.
(Other definitions for distinction that I've seen before include "top exam grade" , "difference" .)