Reduced opening for staff symbol (4)

I believe the answer is:

clef

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'staff symbol' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(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?" .)

I've seen this clue in The Telegraph.

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