Barnum stands in arena’s centre, lying (9)
I believe the answer is:
prostrate
'lying' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for prostrate that I've seen before include "Lying face downwards" , "Lying stretched out" , "helpless" , "no longer able to stand" , "Prone" .)