The most common and most likely answer to this crossword clue is the 11 plus 4 letter word FASHIONABLY LATE
I've seen this crossword clue in Financial Times
F
A
S
H
I
O
N
A
B
L
Y
L
A
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E
Buddy explains!
The straight part
We know that "Attending party? Not before time, staying in" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case attending party? not before time, staying in = fashionably late.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.