Symbol on a stave (4)
I believe the answer is:
clef
'symbol on a stave' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for clef that I've seen before include "Mark determining musical pitch of notes" , "Stave character" , "Pitch symbol on musical stave" , "Musical key or stave" , "Musical notation on a staff" .)