Nanny's nightmare (4)
I believe the answer is:
brat
'nanny's nightmare' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for brat that I've seen before include "Bad child - pack member!" , "annoying young fellow" , "Marie ---, birth-control pioneer" , "Horrible child" , "Troublesome child" .)