The most common and most likely answer to this crossword clue is the 5 plus 8 letter word FIFTH BIRTHDAY
F
I
F
T
H
B
I
R
T
H
D
A
Y
Buddy explains!
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.