One foresees those royals getting upset when left out (10)

I believe the answer is:

soothsayer

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'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

I can't explain the remainder of the clue.

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(Other definitions for soothsayer that I've seen before include "Prophet" , "forecaster" , "One foreseeing future" , "He warned Caesar" .)

I've seen this clue in The Irish Times.

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