The most common and most likely answer to this crossword clue is the 9 letter word PANGOLINS
I've seen this crossword clue in The Daily Telegraph
P
A
N
G
O
L
I
N
S
Buddy explains!

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 = pangolins.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 PANGOLINS mean?
PANGOLINS noun- The scaly anteater; any of several long-tailed, scale-covered mammals of the order Pholidota of tropical Africa and Asia, the sole extant genus of which is Manis.