The most common and most likely answer to this crossword clue is the 4 letter word KUDU
I've seen this crossword clue in The Guardian
K
U
D
U
Buddy explains!
The straight part
We know that "Ruminant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ruminant = kudu.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 KUDU mean?
KUDU noun- A large, striped, African antelope of the species Tragelaphus imberbis (the lesser kudu) or Tragelaphus strepsiceros (the greater kudu).