The most common and most likely answer to this crossword clue is the 7 letter word LEFTIES
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Buddy explains!

The straight part
We know that "sinister people" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sinister people = lefties.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 LEFTIES mean?
LEFTIES noun- One who is left-handed.
- One who has left-wing political views.
- One's left testicle.