Childish affliction initially resolved in major success (5)
I believe the answer is:
croup
'childish affliction' is the definition.
'croup' can be an answer for 'affliction' (croup is a kind of health problem). I am unsure of the 'childish' bit.
'initially resolved in major success' is the wordplay.
'initially' says to take the initial letters.
'in' is an insertion indicator.
'major success' becomes 'coup' (I've seen this in another clue).
The first letter of 'resolved' is 'r'.
'r' going into 'coup' is 'CROUP'.
Can you help me to learn more?
(Other definitions for croup that I've seen before include "Throat infection; horse's hindquarters" , "Hard cough and difficult breathing in children" , "Infantile breathing difficulties" , "Rump of a horse" , "Inflammation of the larynx and trachea in children" .)