High-class sound? (4)

I believe the answer is:

tone

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'sound?' is the definition.
(I know that tone is a type of sound)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for tone that I've seen before include "Note; note (anag.)" , "Pitch of a voice" , "Quality or character of sound" , "Vocal sound of note" , "Quality of a person's voice" .)

I've seen this clue in The Irish News.

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