The most common and most likely answer to this crossword clue is the 12 letter word BICENTENNIAL
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Buddy explains!

The straight part
We know that "a certain anniversary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a certain anniversary = bicentennial.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 BICENTENNIAL mean?
BICENTENNIAL noun- The 200th anniversary of an event or happening.
- Relating to the 200th anniversary of an event or happening.
- Occurring every two hundred (200) years