Ill-mannered child (4)
I believe the answer is:
brat
'ill-mannered child' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for brat that I've seen before include "Brash young type" , "Bad child - pack member!" , "Spoilt kid" , "child - that's a nuisance" , "Marie ---, birth-control pioneer" .)