One's second self (5,3)
I believe the answer is:
alter ego
'one's second self' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for alter ego that I've seen before include "A second personality or intimate friend" , "'One's alternative personality (5,3)'" , "Mister Hyde, for example" , "Hidden or second self - in Latin" , "bosom buddy" .)