The most common and most likely answer to this crossword clue is the 6 letter word WORSTS
I've seen this crossword clue in Independent
W
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R
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Buddy explains!

The straight part
We know that "Vanquishes/most terrible things" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case vanquishes/most terrible things = worsts.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 WORSTS mean?
WORSTS noun- Something or someone that is the worst
- To make worse.
- To grow worse; to deteriorate.
- To outdo or defeat, especially in battle.