Id's partner (3)
I believe the answer is:
ego
'id's partner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ego that I've seen before include "Morale" , "(Person's) sense of self-esteem" , "Self-esteem, conceit" , "One's self-image" , "Sense of identity" .)