Musical pitch indicator (4)
I believe the answer is:
clef
'musical pitch indicator' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for clef that I've seen before include "Staff note" , "Symbol in musical score showing pitch of the notes" , "Written musical symbol indicating pitch" , "Pitch symbol on musical stave" , "Symbol on the left of a musical stave" .)