Bass or treble (4)
I believe the answer is:
clef
'bass or treble' 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 stave sign" , "Musical pitch indicator" , "Item of musical notation" , "Written musical symbol indicating pitch" , "Symbol on stave in musical score" .)