Change, say, when meeting 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 when meeting old' is the wordplay.
'change' becomes 'alter' (synonyms).
'say' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'when meeting' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'alter'+'eg'+'o'='ALTER EGO'
(Other definitions for alter ego that I've seen before include "Other self - large toe (anag)" , "bosom buddy" , "To regale other self" , "Hidden or second self - in Latin" , "'One's second self (5,3)'" .)