The most common and most likely answer to this crossword clue is the 5 plus 8 plus 2 plus 3 letter word HAPPY BIRTHDAY TO YOU
H
A
P
P
Y
B
I
R
T
H
D
A
Y
T
O
Y
O
U
Buddy explains!
The straight part
We know that "number" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case number = happy birthday to you.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.