Mistake beheading troublesome brat (5)
I believe the answer is:
error
'mistake' is the definition.
(I know that mistake can be written as error)
'beheading troublesome brat' is the wordplay.
'beheading' means to remove the first letter.
'troublesome brat' becomes 'terror' (term for a misbehaving child).
'terror' with its initial letter removed is 'ERROR'.
(Other definitions for error that I've seen before include "Faux pas" , "Blunder or mistake" , "Boob" , "Slip-up" , "Gaffe" .)