First impression that establishes the truth (5)

I believe the answer is:

proof

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'first impression' is the definition.
'proof' can be an answer for 'impression' (proof is a kind of impression). I'm unsure of the 'first' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for proof that I've seen before include "whisky's strength?" , "Specimen" , "Incontrovertible evidence" , "Strength of spirit" , "a writer may check" .)

I've seen this clue in The Irish Independent.

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