Splendid personality in part of unconscious mind (8)
I believe the answer is:
superego
'part of unconscious mind' is the definition.
('superego' is part of 'unconscious mind')
'splendid personality' is the wordplay.
'splendid' becomes 'super' (I've seen this before).
'personality' becomes 'ego' (I've seen this before).
'super'+'ego'='SUPEREGO'
'in' acts as a link.
(Other definitions for superego that I've seen before include "Freud's part of the mind that acts as a self-critical conscience" , "piece of his mind?" , "Conscience, perhaps" , "Moral part of the (Freudian) unconscious" , "Internal monitor of psyche" .)