The most common and most likely answer to this crossword clue is the 10 letter word BRONTOSAUR
I've seen this crossword clue in Financial Times
B
R
O
N
T
O
S
A
U
R
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 = brontosaur.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 BRONTOSAUR mean?
BRONTOSAUR noun- Any member of the genus Brontosaurus.