Music score symbol (4)
I believe the answer is:
clef
'music score symbol' 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 pitch indicator" , "Musical character" , "Musical key or stave" , "Symbol in musical score showing pitch of the notes" , "Treble or bass sign?" .)