Tearaway ultimately at fault (6)
I believe the answer is:
terror
'tearaway' is the definition.
(in the sense of a misbehaving child)
'ultimately at fault' is the wordplay.
'ultimately' indicates one should take the final letters.
'fault' becomes 'error' ('error' can be a synonym of 'fault').
The final letter of 'at' is 't'.
't'+'error'='TERROR'
(Other definitions for terror that I've seen before include "Extreme fear of dread" , "Young miscreant" , "Extreme fright or dread" , "Panicky fright" , "Troublesome child" .)