The most common and most likely answer to this crossword clue is the 9 letter word MASTERFUL
I've seen this crossword clue in The Times
M
A
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More clues leading to the result MASTERFUL
- New smart fuel provided by expert
- Arrogant graduates crying when one goes missing
- Like imam that’s authoritative expert
- Imperious scholar worries about university lecturer
- Showing authority with worries mounting during strike
- Expert fusses over dividing batter
- Authoritative ref’s possibly restricting time in feature of rugby
- Dominant in fight, concealing worries to make comeback
- Arrogant male with flower — almost perfect
- Almost complete support for teacher exercising authority
Buddy explains!

The straight part
We know that "expert" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case expert = masterful.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 MASTERFUL mean?
MASTERFUL adjective- Of or pertaining to the manner of a master of an art, technique, profession, or craft; masterly.
- In the manner of a master; very skillful.
- Having or showing the qualities of a master vis-a-vis a dependent or subordinate; authoritative; domineering.