Not for many to know with respect to changing (3-6)
I believe the answer is:
top-secret
'not for many to know' is the definition.
'with respect to changing' is the wordplay.
'with' is a charade indicator (letters next to each other).
'changing' indicates an anagram.
'respect' put next to 'to' is 'respectto'.
'respectto' is an anagram of 'TOP-SECRET'.
(Other definitions for top-secret that I've seen before include "Highly classified" , "Highly restricted (information)" , "Highest classified (information)" , "Extremely restricted (information)" , "Most confidential , not to be leaked on any account" .)