The most common and most likely answer to this crossword clue is the 7 plus 4 letter word HIGHWAY CODE
I've seen this crossword clue in The Guardian
H
I
G
H
W
A
Y
C
O
D
E
Buddy explains!
The straight part
We know that "Flyover etiquette/guide for road users" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flyover etiquette/guide for road users = highway code.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.