Appoint different agent after changing side (9)
I believe the answer is:
designate
'appoint' is the definition.
(designating is a kind of appointing)
'different agent after changing side' is the wordplay.
'different' indicates an anagram.
'after' says to put letters next to each other.
'changing' indicates anagramming the letters.
'agent' with letters rearranged gives 'gnate'.
'side' with letters rearranged gives 'desi'.
'gnate' after 'desi' is 'DESIGNATE'.
(Other definitions for designate that I've seen before include "Allocate" , "Identify" , "Assign" , "Appoint" , "Appointed but not yet in post" .)