The most common and most likely answer to this crossword clue is the 9 letter word BASILISKS
I've seen this crossword clue in The Times
B
A
S
I
L
I
S
K
S
Buddy explains!

The straight part
We know that "Lethal serpents" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lethal serpents = basilisks.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 BASILISKS mean?
BASILISKS noun- A mythical (and heraldic) snake-like dragon type, reputed to be so venomous that its gaze was deadly.
- A type of dragon used in heraldry.
- A tree-dwelling type of lizard of the genus Basiliscus - the basilisk lizard
- A type of large brass cannon.