The most common and most likely answer to this crossword clue is the 10 letter word VIVIPAROUS
I've seen this crossword clue in Financial Times
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More clues leading to the result VIVIPAROUS
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The straight part
We know that "like most mammals" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like most mammals = viviparous.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 VIVIPAROUS mean?
VIVIPAROUS adjective- (of an animal or animal species) Being born alive, as are most mammals, some reptiles, and a few fish (as opposed to being laid as an egg and subsequently hatching, as do most birds and many other species).
- (of a plant or plant species) Arising from an embryo that develops from the outset (rather than from a true seed that then germinates).