clef vs viola clef

clef and viola clef both are nouns.

Word	Noun	Adjective	Verb	Adverb
clef	Yes	No	No	No
viola clef	Yes	No	No	No

As nouns, viola clef is a hyponym of clef; that is, viola clef is a word with a more specific, narrower meaning than clef:

clef: a musical notation written on a staff indicating the pitch of the notes following it
viola clef: a clef that puts middle C on the third line of a staff

clef (noun)	viola clef (noun)
a musical notation written on a staff indicating the pitch of the notes following it	a clef that puts middle C on the third line of a staff