The most common and most likely answer to this crossword clue is the 13 letter word GLORIFICATION
I've seen this crossword clue in Independent
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Buddy explains!
The straight part
We know that "festivity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case festivity = glorification.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 GLORIFICATION mean?
GLORIFICATION noun- The act of glorifying or the state of being glorified.
- Specifically, the ascension (of Christ or humans) to the glory of heaven.
- The worshiping of a deity; extolment or laudation.
- The portrayal of something as being ideal; idealization.