The most common and most likely answer to this crossword clue is the 10 letter word BREAST-FED
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More clues leading to the result BREAST-FED
Buddy explains!

The straight part
We know that "Maybe mammals are" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case maybe mammals are = breast-fed.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 BREAST-FED mean?
BREAST-FED adjective- That has been suckled at its mother's breast
- To feed (a baby) milk via the breasts; to suckle; to nurse.
- (chiefly of a baby) To nurse, to suck milk from a breast.