Pre-adolescent child, affectedly quaint, developed anger (9)
I believe the answer is:
tweenager
'pre-adolescent child' is the definition.
(tweenager can mean a young teenager)
'affectedly quaint developed anger' is the wordplay.
'affectedly quaint' becomes 'twee' (twee can mean quaint or cute).
'developed' is an anagram indicator.
'anger' anagrammed gives 'nager'.
'twee'+'nager'='TWEENAGER'
(Other definitions for tweenager that I've seen before include "exasperating twelve-year-old" , "Precocious child" , "adolescent" , "Youngster" , "(Informally) a child approaching the age of 13?" .)