The most common and most likely answer to this crossword clue is the 9 letter word MURDEROUS
I've seen this crossword clue in The Guardian
M
U
R
D
E
R
O
U
S
More clues leading to the result MURDEROUS
Buddy explains!

The straight part
We know that "Lethal/Lethal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lethal/lethal = murderous.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 MURDEROUS mean?
MURDEROUS adjective- Of, characterized by, or pertaining to murder or murderers.
- Of a person: intending or likely to commit murder; bloodthirsty, homicidal.
- Of an object: used to commit murder; capable of causing death; deadly, fatal.
- (by extension) Very difficult.