Ultimately not mistake to see troublesome child (6)
I believe the answer is:
terror
'troublesome child' is the definition.
(I've seen this in another clue)
'ultimately not mistake' is the wordplay.
'ultimately' indicates one should take the final letters.
'mistake' becomes 'error' (synonyms).
The final letter of 'not' is 't'.
't'+'error'='TERROR'
'to see' acts as a link.
(Other definitions for terror that I've seen before include "Overwhelming feeling of fear" , "Sheer fright" , "Utter dread" , "Little brat" , "Organised political violence" .)