The most common and most likely answer to this crossword clue is the 6 letter word RACISM
I've seen this crossword clue in The Times
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Buddy explains!
The straight part
We know that "unacceptable prejudice" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unacceptable prejudice = racism.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 RACISM mean?
RACISM noun- Belief that there are distinct human races with inherent differences which determine their abilities, and generally that some are superior and others inferior.
- The policies, practices, or systems (e.g. government or political) promoting this belief or promoting the dominance of one or more races over others.
- Prejudice or discrimination based upon race or ethnicity; an action of such discrimination.