The most common and most likely answer to this crossword clue is the 4 letter word CLEF
I've seen this crossword clue in The Guardian
C
L
E
F
More clues leading to the result CLEF
- Division reduced? Sign on staff
- Key article, first to be cut
- Character on staff nearly broken
- Will one fix score on pitch? Au contraire
- Character on staff almost split
- Short division? Sign on staff
- Key section of Article 50
- Symbol of Conservative and most of opposition?
- Character in Cycle finale
- Musical character to have on the staff
Buddy explains!

The straight part
We know that "Sign on staff" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sign on staff = clef.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.
What does CLEF mean?
CLEF noun- A symbol found on a musical staff that indicates the pitches represented by the lines and the spaces on the staff