The most common and most likely answer to this crossword clue is the 8 letter word PARTYING
I've seen this crossword clue in Independent
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Buddy explains!
The straight part
We know that "Having fun at celebration" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case having fun at celebration = partying.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 PARTYING mean?
PARTYING verb- To celebrate at a party, to have fun, to enjoy oneself.
- To take recreational drugs.
- To engage in flings, to have one-night stands, to sow one's wild oats.
- To form a party (with).