Musical pitch indicator (4)
I believe the answer is:
clef
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">musical pitch indicator</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)