Younger son acted wrongly (5)
I believe the answer is:
cadet
'younger son' is the definition.
(archaic term for a younger child)
'acted wrongly' is the wordplay.
'wrongly' indicates an anagram.
'acted' with letters rearranged gives 'CADET'.
(Other definitions for cadet that I've seen before include "Trainee cop" , "Police or armed forces trainee" , "young soldier" , "Apprentice officer" , "Officer in training" .)