Animal on which Darwin rode, starting out (5)

I believe the answer is:

eagle

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'animal' is the definition.
(eagle is a kind of animal)

I cannot understand how the rest of the clue works.

(Other definitions for eagle that I've seen before include "Keen-sighted bird of prey" , "kind of bird" , "King of birds" , "Bird of prey; former children's comic" , "may be soaring" .)

I've seen this clue in The Guardian.

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