strength of spirit (6)
I believe the answer is:
mettle
'strength of spirit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mettle that I've seen before include "Temperament; courage" , "Quality" , "Courage sounds like iron, for instance" , "boldness" , "'Nerve, heart, courage (6)'" .)