The most common and most likely answer to this crossword clue is the 12 letter word HARE-BRAINED
I've seen this crossword clue in The Times
H
A
R
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B
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Buddy explains!
The straight part
We know that "Foolish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case foolish = hare-brained.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 HARE-BRAINED mean?
HARE-BRAINED adjective- (of an idea or plan etc) Absurd, foolish or stupid.
- (of a person) Frivolous and silly; featherbrained or scatterbrained.