The most common and most likely answer to this crossword clue is the 9 letter word DEATHBLOW
I've seen this crossword clue in The Daily Telegraph
D
E
A
T
H
B
L
O
W
More clues leading to the result DEATHBLOW
Buddy explains!

The straight part
We know that "lethal buffet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lethal buffet = deathblow.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 DEATHBLOW mean?
DEATHBLOW noun- A strike or blow that leads to death, especially a coup de grace.
- Something that prevents the completion, or ends the existence, of a project etc.; a fatal setback.