Crossword Genius

*Mammal that can fold its nostrils shut when submerged (12)

Ross

I believe the answer is:

hippopotamus

(Other definitions for hippopotamus that I've seen before include "Oppo hams it up (anag)" , "Creature" , "Large aquatic beast" , "Thick-skinned herbivorous animal of tropical Africa" , "Large African mammal frequenting rivers" .)

I've seen this clue in The Wall Street Journal.
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