That of a true hero is never broken (5)
I believe the answer is:
nerve
'that of a true hero' is the definition.
I can't judge whether this definition defines the answer.
'never broken' is the wordplay.
'broken' indicates anagramming the letters.
'never' with letters rearranged gives 'NERVE'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for nerve that I've seen before include "Courage - cheek" , "Courage, like bodily fibre" , "Sensitive fibre in the body" , "Bundle of fibres < in face" , "Boldness" .)