Tusked cetacean (7)

I believe the answer is:

narwhal

(Other definitions for narwhal that I've seen before include "Arctic whale, the male having a long spiral ivory tusk" , "Arctic toothed whale" , "Marine mammal, the male having a spiral ivory tusk" , "aquatic mammal" , "swimmer" .)

I've seen this clue in the Sydney Morning Herald.

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