The most common and most likely answer to this crossword clue is the 9 letter word TRIENNIAL
I've seen this crossword clue in Financial Times
T
R
I
E
N
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A
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More clues leading to the result TRIENNIAL
Buddy explains!

The straight part
We know that "regular event" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case regular event = triennial.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 TRIENNIAL mean?
TRIENNIAL noun- A third anniversary.
- A plant that requires three years to complete its life-cycle.
- Happening every three years.
- Lasting for three years.