Prodigy's flaw ultimately: not good enough! (10)
I believe the answer is:
wunderkind
'prodigy's' is the definition.
(I've seen this in another clue)
'flaw ultimately not good enough' is the wordplay.
I cannot really understand how this works, but
'not' could be 'un' (prefix meaning 'not') and 'un' is present in the answer.
'good' could be 'kind' (similar in meaning) and 'kind' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for wunderkind that I've seen before include "One having early success" , "Child prodigy" , "Extraordinary issue" , "Perhaps Mozart" , "Clever young person" .)