In favour of conclusive evidence (5)

I believe the answer is:

proof

'conclusive evidence' is the definition.
(I've seen this in another clue)

'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 "Specimen" , "Convincing evidence" , "mathematical tour de force" , "to give a watertight case?" , "may need amending" .)

I've seen this clue in the Evening Standard.

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