The most common and most likely answer to this crossword clue is the 15 letter word TYRANNOSAURUSES
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Buddy explains!
The straight part
We know that "giant reptiles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case giant reptiles = tyrannosauruses.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 TYRANNOSAURUSES mean?
TYRANNOSAURUSES noun- A large carnivorous dinosaur, of the genus Tyrannosaurus, found in North America during the late Cretaceous period.