Crossword Genius

One’s written on British poverty before beast died (10)

Ross

I believe the answer is:

blackboard

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

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

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(Other definitions for blackboard that I've seen before include "Teaching aid" , "where he might chalk up a win" , "something to write on" , "What's written on in the classroom" , "school equipment" .)

I've seen this clue in The Times.
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