Code prince changed about His Excellency (8)
I believe the answer is:
encipher
'code' is the definition.
(synonyms)
'prince changed about his excellency' is the wordplay.
'changed' is an anagram indicator.
'about' is an insertion indicator.
'his excellency' becomes 'he'.
'prince' is an anagram of 'encipr'.
'encipr' placed around 'he' is 'ENCIPHER'.
(Other definitions for encipher that I've seen before include "Convert ordinary language into code" , "Convert to coded form" , "Put into code" .)