Crossword Genius

Barnum stands in arena’s centre, lying (9)

Ross

I believe the answer is:

prostrate

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'lying' is the definition.
(I've seen this before)

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

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(Other definitions for prostrate that I've seen before include "Lying face downwards" , "Lying stretched out" , "helpless" , "no longer able to stand" , "Prone" .)

I've seen this clue in the Sydney Morning Herald.
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