Courage to carry on (6)
I believe the answer is:
mettle
'courage to carry on' 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 "Quality" , "'Nerve, heart, courage (6)'" , "Inherent character - courage" , "Pluck" , "Spirit and courage sound like iron, say" .)