Evidence of spiritual strength (5)

I believe the answer is:

proof

I'm a little stuck... Click here to teach me more about this clue!

'spiritual strength' is the definition.
'proof' can be an answer for 'strength' (I've seen this before). I am not certain of the 'spiritual' bit.

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

Can you help me to learn more?

(Other definitions for proof that I've seen before include "Draft copy of printed text" , "Testimony" , "Decisive evidence" , "Measure of the strength of alcohol" , "Evidence that establishes the truth" .)

I've seen this clue in The Guardian and The Globe and Mail.

Want a hint initially instead of a full solution? Install my app