The most common and most likely answer to this crossword clue is the 7 letter word ONDATRA
O
N
D
A
T
R
A
More clues leading to the result ONDATRA
Buddy explains!
The straight part
We know that "aquatic mammal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case aquatic mammal = ondatra.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.