The most common and most likely answer to this crossword clue is the 8 letter word WALRUSES
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More clues leading to the result WALRUSES
Buddy explains!

The straight part
We know that "Sea mammals" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sea mammals = walruses.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does WALRUSES mean?
WALRUSES noun- A large Arctic marine mammal related to seals and having long tusks, tough, wrinkled skin, and four flippers, Odobenus rosmarus.