A second self (5,3)
I believe the answer is:
alter ego
'a second self' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alter ego that I've seen before include "A person's secondary personality" , "'One's alternative personality (5,3)'" , "Other half" , "One's secondary personality" , "'One's second self (5,3)'" .)