Black rodent irritating child (4)
I believe the answer is:
brat
'irritating child' is the definition.
(term for a misbehaving child)
'black rodent' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'rodent' becomes 'rat' (rat is a kind of rodent).
'b'+'rat'='BRAT'
(Other definitions for brat that I've seen before include "Spoilt child" , "Nasty little kid" , "Annoying child" , "Nasty child" , "An obnoxious or spoiled child" .)