The most common and most likely answer to this crossword clue is the 3 letter word GNU
I've seen this crossword clue in The Times
G
N
U
More clues leading to the result GNU
- Animal jumping numerous hedges
- African nomad's regular display of genius
- Gun shot big beast
- Antelope starts to get nervous, uneasy
- Antelope found in growing numbers
- Animal eating nuts, but not the whole lot
- Wildebeest seen in big numbers
- Beasts recorded in dwindling numbers
- Animal found in alarming numbers
- Antelope bagged by foreign upstart
Buddy explains!

The straight part
We know that "big beast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case big beast = gnu.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 GNU mean?
GNU noun- A large antelope of the genus Connochaetes, native to Africa, having curved horns.