The most common and most likely answer to this crossword clue is the 8 letter word STULTIFY
I've seen this crossword clue in The Guardian
S
T
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More clues leading to the result STULTIFY
Buddy explains!

The straight part
We know that "Cause to look foolish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cause to look foolish = stultify.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 STULTIFY mean?
STULTIFY verb- To prove to be of unsound mind or demonstrate someone's incompetence.
- To cause to appear foolish.
- To deprive of strength or efficacy; make useless or worthless.