The most common and most likely answer to this crossword clue is the 11 letter word CERTIFICATE
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Buddy explains!
The straight part
We know that "official declaration" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case official declaration = certificate.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 CERTIFICATE mean?
CERTIFICATE noun- A document containing a certified statement.
- A document evidencing ownership or debt.
- A document serving as evidence as a person has completed an educational course, issued either by an institution not authorised to grant diplomas, or to a student not qualifying for a diploma.
- The information needed in order to verify a positive answer to a problem.
- A motion picture age rating.
- To supply with a certificate, especially following certification