The most common and most likely answer to this crossword clue is the 8 letter word NARWHALS
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The straight part
We know that "toothy mammals" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case toothy mammals = narwhals.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 NARWHALS mean?
NARWHALS noun- Monodon monoceros, an Arctic cetacean that grows to about 20 feet (6 meters) long, the male having a single horn-like tusk, a twisted, pointed canine tooth that projects forward.