Evidence of strength of spirit (5)

I believe the answer is:

proof

'strength of spirit' is the definition.
(I've seen this before)

'evidence of' is the wordplay.
'evidence' becomes 'pro' (pro is a kind of evidence).
'pro'+'of'='PROOF'

(Other definitions for proof that I've seen before include "Clear evidence" , "Testimony" , "mathematical work" , "Decisive evidence" , "Incontrovertible evidence" .)

I've seen this clue in The Daily Express.

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