Scottish youngster’s brain a little affected (5)
I believe the answer is:
bairn
'scottish youngster's' is the definition.
(Scottish term for a child)
'brain a little affected' is the wordplay.
'a little affected' is an anagram indicator.
'brain' is an anagram of 'BAIRN'.
(Other definitions for bairn that I've seen before include "Scot. child" , "A Scottish child?" , "Northern child" , "Child in Scotland" , "little one in Yorkshire" .)