The most common and most likely answer to this crossword clue is the 6 letter word NO-NOS
N
O
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Buddy explains!

The straight part
We know that "they’re taboo" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they’re taboo = no-nos.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 NO-NOS mean?
NO-NOS noun- Something that is forbidden, prohibited, discouraged or taboo.
- A no-hitter; a game in which no batter on one of the teams got a hit.