The most common and most likely answer to this crossword clue is the 4 letter word GOAT
I've seen this crossword clue in The Guardian
G
O
A
T
More clues leading to the result GOAT
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 = goat.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 GOAT mean?
GOAT noun- A mammal, Capra aegagrus hircus, and similar species of the genus Capra.
- A lecherous man.
- A scapegoat.
- A Pontiac GTO car.
- (speech recognition) A person who is not easily understood by a speech recognition system; contrasted with sheep.
- A fool, loser, or object of ridicule.
- To allow goats to feed on.
- To scapegoat.