The most common and most likely answer to this crossword clue is the 3 plus 3 plus 5 plus 2 plus 3 letter word YAH BOO SUCKS TO YOU
I've seen this crossword clue in Financial Times
Y
A
H
B
O
O
S
U
C
K
S
T
O
Y
O
U
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 = yah boo sucks to you.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.