Unconcealed single part of harmonic series (8)

I believe the answer is:

overtone

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'part of harmonic series' is the definition.
(I've seen this before)

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

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(Other definitions for overtone that I've seen before include "Subtle implication or connotation" , "Nuance" , "Subtle quality" , "Hint" , "Suggestion" .)

I've seen this clue in The Sunday Times and The Times.

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