Trusted friend to change and say nothing (5,3)
I believe the answer is:
alter ego
'trusted friend' is the definition.
(alter ego can informally mean a close friend)
'change and say nothing' is the wordplay.
'change' becomes 'alter' (synonyms).
'and' is a charade indicator (letters next to each other).
'say' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'nothing' becomes 'o' (looks like zero - 0).
'alter'+'eg'+'o'='ALTER EGO'
'to' acts as a link.
(Other definitions for alter ego that I've seen before include "To regale other self" , "bosom buddy" , "'One's second self (5,3)'" , "Change conceit for other self" , "Mister Hyde, for example" .)