The most common and most likely answer to this crossword clue is the 5 letter word CANID
I've seen this crossword clue in The Guardian
C
A
N
I
D
Buddy explains!

The straight part
We know that "Mammal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mammal = canid.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 CANID mean?
CANID noun- Any member of the family Canidae, including dogs, wolves, foxes, coyotes and jackals.