Musician uses it, partially, to build muscle fibre (4)
I believe the answer is:
clef
'musician' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'it partially to build muscle fibre' is the wordplay.
'CLEF' is hidden within the letters.
But, I'm uncertain how this is indicated.
'uses' is the link.
Can you help me to learn more?
(Other definitions for clef that I've seen before include "Treble or bass on a stave" , "Musical key or stave" , "Symbol written on sheet music to indicate pitch" , "Symbol on stave in musical score" , "Treble or bass symbol?" .)