The most common and most likely answer to this crossword clue is the 4 plus 6 plus 9 letter word NINA PRETTY BALLERINA
I've seen this crossword clue in The Guardian
N
I
N
A
P
R
E
T
T
Y
B
A
L
L
E
R
I
N
A
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 = nina pretty ballerina.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.