Strength of spirit in demonstration (5)
I believe the answer is:
proof
This clue is a double definition.
'strength of spirit' is the first definition.
(I've seen this before)
'demonstration' is the second definition.
(I've seen this in another clue)
'in' is the link.
(Other definitions for proof that I've seen before include "Demonstration of the truth of something" , "Author may have to check this" , "mathematical tour de force" , "Conclusive evidence or verification" , "Convincing evidence" .)