Musical sign (4)
I believe the answer is:
clef
'musical sign' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for clef that I've seen before include "Musical character" , "Pitch symbol on musical stave" , "Symbol written on sheet music to indicate pitch" , "Musical stave sign" , "Symbol on the left of a musical stave" .)