The most common and most likely answer to this crossword clue is the 8 letter word SERPENTS
I've seen this crossword clue in The Guardian
S
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R
P
E
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S
More clues leading to the result SERPENTS
Buddy explains!

The straight part
We know that "Reptiles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reptiles = serpents.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 SERPENTS mean?
SERPENTS noun- A snake.
- An obsolete wind instrument in the brass family, whose shape is suggestive of a snake (Wikipedia article).
- A subtle, treacherous, malicious person.
- A kind of firework with a serpentine motion.
- To wind or meander
- To encircle.