Reduced opening for staff symbol (4)

I believe the answer is:
clef
'staff symbol' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for clef that I've seen before include "Symbol on stave in musical score" , "Musical pitch notation" , "Musical stave sign" , "Musical pitch indicator" , "Treble or bass symbol?" .)
