Change, say, then ring one's closest friend (5,3)
I believe the answer is:
alter ego
'one's closest friend' is the definition.
(alter ego can informally mean a close friend)
'change say then ring' is the wordplay.
'change' becomes 'alter' (altering is a kind of changing).
'say' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'then' says to put letters next to each other.
'ring' becomes 'o' (letter resembling a ring).
'alter'+'eg'+'o'='ALTER EGO'
(Other definitions for alter ego that I've seen before include "Other half" , "Hidden or second self - in Latin" , "Is it Kent" , "To regale other self" , "A person's secondary personality" .)