Second self (5,3)
I believe the answer is:
alter ego
'second self' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alter ego that I've seen before include "Other half" , "'One's alternative personality (5,3)'" , "Go, relate to other self" , "Hidden or second self - in Latin" , "Mister Hyde, for example" .)