100 as old as us showing mettle (7)
I believe the answer is:
courage
'mettle' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for courage that I've seen before include "Bravery, heroism" , "Grit" , "Bravery, fortitude" , "Ability to conquer fear or despair" , "Gallantry" .)