The most common and most likely answer to this crossword clue is the 10 letter word PLATYPUSES
I've seen this crossword clue in The Guardian
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The straight part
We know that "Mammals" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mammals = platypuses.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 PLATYPUSES mean?
PLATYPUSES noun- A semi-aquatic, egg-laying monotreme mammal with a bill resembling that of a duck, that has a mole-like body, a tail resembling that of a beaver, a waterproof pelt, and flat webbed feet — males have poisonous spurs on the inside of the back legs; Ornithorhynchus anatinus