It's not good for a legendary hero to be faultless! (3)
I believe the answer is:
bad
'it's not good' is the definition.
(bad is the opposite of good)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for bad that I've seen before include "SECOND HALF OF 16" , "Ill" , "Rotten, evil" , "Severe; of low standard" , "Substandard" .)