Change, say, before seeing old trusted friend (5,3)
I believe the answer is:
alter ego
'trusted friend' is the definition.
(alter ego can informally mean a close friend)
'change say before seeing old' is the wordplay.
'change' becomes 'alter' (altering is a kind of changing).
'say' becomes 'eg' (both can mean 'for example').
'before' is a charade indicator (letters next to each other).
'seeing old' becomes 'o' (abbreviation).
'alter'+'eg'+'o'='ALTER EGO'
(Other definitions for alter ego that I've seen before include "Change conceit for other self" , "Similar character" , "Eg, to earl? (Anag)" , "Is it Kent" , "A second personality or intimate friend" .)