The most common and most likely answer to this crossword clue is the 2 plus 9 letter word NO ADMISSION
I've seen this crossword clue in Independent
N
O
A
D
M
I
S
S
I
O
N
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.