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Biologist atop swaying tree - one may be out on the field (9)

Ross

I believe the answer is:

cricketer

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'one may be out on the field' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for cricketer that I've seen before include "Point one may be at" , "Don Bradman or David Gower, say" , "Sportsman who bats or bowls" , "Player of traditional English game" , "Paul Stirling or William Porterfield, say" .)

I've seen this clue in The Telegraph.
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