Fear brat's latest mistake (6)
I believe the answer is:
terror
'fear' is the definition.
(terror is a kind of fear)
'brat's latest mistake' is the wordplay.
'latest' indicates one should take the final letters.
'mistake' becomes 'error' (synonyms).
The last letter of 'brat' is 't'.
't'+'error'='TERROR'
(Other definitions for terror that I've seen before include "Overwhelming feeling of fear" , "Ovewhelming fear" , "Young rascal -- horror" , "Out-of-control child" , "State of great fright" .)