In favour of demonstration? (5)

I believe the answer is:

proof

'demonstration?' is the definition.
(demonstration is a kind of proof)

'in favour of' is the wordplay.
'in favour' becomes 'pro' (I've seen this before).
'pro'+'of'='PROOF'

(Other definitions for proof that I've seen before include "to give a watertight case?" , "whisky's strength?" , "Strength of spirit" , "Factual evidence" , "Trial printing" .)

I've seen this clue in The Independent.

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