The most common and most likely answer to this crossword clue is the 3 letter word AGM
I've seen this crossword clue in Financial Times
A
G
M
Buddy explains!
The straight part
We know that "Once-yearly/ gathering" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case once-yearly/ gathering = agm.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.