The most common and most likely answer to this crossword clue is the 11 letter word HALF-WITTED
I've seen this crossword clue in The Guardian
H
A
L
F
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W
I
T
T
E
D
More clues leading to the result HALF-WITTED
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 = half-witted.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 HALF-WITTED mean?
HALF-WITTED adjective- Having a low intelligence; dim-witted