Labelling piece of jewelry worn by mother round back of neck (10)

I believe the answer is:

earmarking

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'labelling' is the definition.
The answer and definition can be both related to thinking as well as being present participle verbs.
Perhaps you can see a link between them that I can't see?

I cannot understand how the remainder of the clue works.

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(Another definition for earmarking that I've seen is " Designating".)

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

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