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Sea creature managed to surface with truncated sound (7)

Ross

I believe the answer is:

narwhal

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'sea creature' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps there's an association between them I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for narwhal that I've seen before include "Small Arctic whale" , "aquatic mammal" , "Tusked marine creature" , "Breed of small whale" , "Arctic whale, the male having a long spiral ivory tusk" .)

I've seen this clue in The Telegraph.
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