The most common and most likely answer to this crossword clue is the 7 letter word ASININE
I've seen this crossword clue in The Times
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N
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More clues leading to the result ASININE
- A serious error in English is foolish
- Very stupid and insane possibly — one must be kept in
- Foolish fiend regularly gate-crashes a function
- Stupid, like idiot’s opening figure
- Stupid, a transgression in office at the end
- How to label square off the top-right of a chessboard? That’s idiotic
- . A transgression that’s consuming knight is silly
- Vacuous account about odd figure
- Idiotic square like one before
- When one over the eight, one’s drunk and silly
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 = asinine.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 ASININE mean?
ASININE adjective- Very foolish; failing to exercise intelligence or judgement or rationality
- Of, pertaining to, or characteristic of donkeys