The most common and most likely answer to this crossword clue is the 5 letter word DIVVY
I've seen this crossword clue in The Times
D
I
V
V
Y
More clues leading to the result DIVVY
Buddy explains!
The straight part
We know that "Whack idiot" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case whack idiot = divvy.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 DIVVY mean?
DIVVY noun- A dividend; a share or portion.
- To divide into portions.
- A foolish person.
- Divine; very pleasant, wonderful.