A child with autism exhibits initially different aspects of their character (8)
I believe the answer is:
personae
'different aspects of their character' is the definition.
I can't tell whether this definition defines the answer.
'a child with autism exhibits initially' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'child' becomes 'son' (male child).
'with' says to put letters next to each other.
'initially' suggests taking the first letters.
The first letters of 'autism exhibits' is 'ae'.
'per'+'son'+'ae'='PERSONAE'
Can you help me to learn more?
(Other definitions for personae that I've seen before include "Dramatis ..., characters in a play" , "See 1" , "Theatrical roles" .)