Unpleasant child (4)
I believe the answer is:
brat
'unpleasant child' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for brat that I've seen before include "Urchin" , "annoying young fellow" , "A spoiled, badly behaved child" , "Nasty little kid" , "Troublesome child" .)