The most common and most likely answer to this crossword clue is the 8 letter word MUSQUASH
I've seen this crossword clue in The Guardian
M
U
S
Q
U
A
S
H
Buddy explains!

The straight part
We know that "Large rodent" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case large rodent = musquash.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 MUSQUASH mean?
MUSQUASH noun- The muskrat, Ondatra zibethicus.