The most common and most likely answer to this crossword clue is the 8 letter word ROE-DEER
I've seen this crossword clue in Financial Times
R
O
E
-
D
E
E
R
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 = roe-deer.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 ROE-DEER mean?
ROE-DEER noun- A small, nimble Eurasian deer with no visible tail, a white rump patch, and a reddish summer coat that turns grey in winter, the male having short three-pointed antlers (Capreolus capreolus and Capreolus pygargus).