The most common and most likely answer to this crossword clue is the 10 letter word UNBIRTHDAY
I've seen this crossword clue in The Guardian
U
N
B
I
R
T
H
D
A
Y
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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 UNBIRTHDAY mean?
UNBIRTHDAY noun- A day that is not one's birthday but is celebrated as though it were.