Mental toughness (4)
I believe the answer is:
grit
'mental toughness' is the definition.
(I know that mental toughness can be written as grit)
This is the entire clue.
(Other definitions for grit that I've seen before include "Persevering courage" , "Determination - small bits of stone" , "Gravelly courage" , "Gravel or colloquial courage" , "Courage (needed by chickens?)" .)