The most common and most likely answer to this crossword clue is the 12 letter word AUTHENTICATE
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The straight part
We know that "Verify" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case verify = authenticate.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does AUTHENTICATE mean?
AUTHENTICATE adjective- Of the same origin as claimed; genuine.
- Conforming to reality and therefore worthy of trust, reliance, or belief.
- (of a Gregorian mode) Having the final as the lowest note of the mode.
- Authoritative
- To render authentic; to give authority to, by the proof, attestation, or formalities required by law, or sufficient to entitle to credit.
- To prove authentic; to determine as real and true.