The most common and most likely answer to this crossword clue is the 4 plus 2 plus 4 letter word WHAT AN IDEA
I've seen this crossword clue in The Guardian
W
H
A
T
A
N
I
D
E
A
Buddy explains!
The straight part
We know that "Preposterous" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case preposterous = what an idea.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.