Crossword Genius

Once spotted hare in play round boy (7)

Ross

I believe the answer is:

bausond

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'once' is the definition.
Both the answer and definition are adjectives.
Perhaps you can see a link between them that I can't see?

I don't understand the rest of the clue.

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(Another definition for bausond that I've seen is " Previously spotted white".)

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