The most common and most likely answer to this crossword clue is the 13 letter word ACCREDITATION
I've seen this crossword clue in The Sunday Telegraph
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Buddy explains!

The straight part
We know that "Official authorising" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case official authorising = accreditation.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.
Crossword Answer definitions
ACCREDITATION noun- The giving of credentials.
- The act of accrediting.
- The granting of approval to an institution of higher learning by an official review board after the school has met certain requirements.