Crossword Genius

One for sorrow among the chattering classes? (6)

Ross

I believe the answer is:

magpie

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'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for magpie that I've seen before include "'One is for sorrow, two for joy (6)'" , "Half of 1D [GAME OR BIRD]" , "chatty bird" , "A black-and-white thieving bird" , "1 [UNLUCKY FOR SOME]" .)

I've seen this clue in The Sun.
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