The most common and most likely answer to this crossword clue is the 9 letter word CANONICAL
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More clues leading to the result CANONICAL
Buddy explains!

The straight part
We know that "Authoritative" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case authoritative = canonical.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 CANONICAL mean?
CANONICAL noun- The formal robes of a priest.
- A URL presented in canonical form.
- Present in a canon, religious or otherwise.
- According to recognised or orthodox rules.
- Stated or used in the most basic and straightforwardly applicable manner.
- Prototypical.
- In conformity with canon law.
- In the form of a canon.
- Of or pertaining to an ecclesiastical chapter
- In canonical form.
- Distinguished among entities of its kind, so that it can be picked out in a way that does not depend on any arbitrary choices.