The most common and most likely answer to this crossword clue is the 2 plus 8 letter word NO ENTRANCE
I've seen this crossword clue in Financial Times
N
O
E
N
T
R
A
N
C
E
Buddy explains!
The straight part
We know that "sign on 24" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sign on 24 = no entrance.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.