The most common and most likely answer to this crossword clue is the 4 plus 4 letter word DROP DEAD
I've seen this crossword clue in The Guardian
D
R
O
P
D
E
A
D
More clues leading to the result DROP DEAD
Buddy explains!
The straight part
We know that "insult" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case insult = drop dead.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.
Crossword Answer definitions
DROP DEAD adverb- Very impressively or spectacularly.
- (of a point in time) Absolutely final; being the deadline.