The most common and most likely answer to this crossword clue is the 10 letter word BLITHERING
I've seen this crossword clue in The Times
B
L
I
T
H
E
R
I
N
G
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 = blithering.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 BLITHERING mean?
BLITHERING verb- To talk foolishly; to blather
- Incoherent or foolish talk.
- Talking incoherently; jabbering.