The most common and most likely answer to this crossword clue is the 8 letter word IMBECILE
I've seen this crossword clue in Independent
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More clues leading to the result IMBECILE
- Fool in island stream, short in distance
- Sap parasites returning to infest wood, edges chewed off
- One doctor picked up infestation on head of envoy, the idiot!
- Dolt? One male celeb, I suspect
- Fool in island stream, unending in distance
- The writer’s spiteful comments about the City may be foolish
- One doctor brought ice back, the French fool
- Foolish Cecilia stripped off without honour
- Stupid one putting insects back among decoration
- I am shortly to be taking part in a council election — what an idiot!
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 = imbecile.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 IMBECILE mean?
IMBECILE noun- A person with limited mental capacity who can perform tasks and think only like a young child, in medical circles meaning a person who lacks the capacity to develop beyond the mental age of a normal five to seven-year-old child.
- A fool, an idiot.
- Destitute of strength, whether of body or mind; feeble; impotent; especially, mentally weak.