Never-failing courage (5)
I believe the answer is:
nerve
'courage' is the definition.
(I know that nerve is a type of courage)
'never-failing' is the wordplay.
'failing' indicates an anagram (a failed spelling of the word).
'never' is an anagram of 'NERVE'.
(Other definitions for nerve that I've seen before include "displaying self-possession" , "Sensor for pain etc" , "Steadiness, courage" , "Strand of body tissue" , "Confidence" .)