The most common and most likely answer to this crossword clue is the 9 letter word MEGAFAUNA
I've seen this crossword clue in Financial Times
M
E
G
A
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Buddy explains!
The straight part
We know that "big animals" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case big animals = megafauna.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 MEGAFAUNA mean?
MEGAFAUNA noun- The large animals of a given region or time, considered as a group.
- A treatise on such a group of large animals.