Vessel a swine almost capsized, circling river (5)

I believe the answer is:

aorta

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'vessel' is the definition.
(aorta is a kind of vessel)

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

(Other definitions for aorta that I've seen before include "Vital blood vessel" , "Major artery of the body" , "Trunk artery from the heart" , "Large trunk artery" , "Large blood-vessel" .)

I've seen this clue in The Independent.

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