Animal’s beginning to swim perfectly (5)

I believe the answer is:

stoat

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But I can't explain it at all! Could you help me understand?

(Other definitions for stoat that I've seen before include "Small carnivorous mammal of the weasel family" , "Source of ermine" , "Member of the weasel family" , "Ermine in summertime?" , "Provider of ermine" .)

I've seen this clue in The Hindu.

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