The most common and most likely answer to this crossword clue is the 8 letter word HUMPBACK
I've seen this crossword clue in The Times
H
U
M
P
B
A
C
K
More clues leading to the result HUMPBACK
Buddy explains!

The straight part
We know that "beast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case beast = humpback.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 HUMPBACK mean?
HUMPBACK noun- A humped back (deformity in humans caused by abnormal curvature of the upper spine).
- A person with a humpback; a person who suffers from kyphosis.
- A humpback whale.
- A humpback salmon.
- To hunt humpback whales.