The most common and most likely answer to this crossword clue is the 7 letter word CATFLAP
I've seen this crossword clue in The Daily Telegraph
C
A
T
F
L
A
P
More clues leading to the result CATFLAP
Buddy explains!

The straight part
We know that "Entry of animal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case entry of animal = catflap.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 CATFLAP mean?
CATFLAP noun- Small hinged panel, usually cut into a door, with an opening just big enough for a cat to enter.