The most common and most likely answer to this crossword clue is the 6 letter word AMPULS
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Buddy explains!

The straight part
We know that "Sealed vessels" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sealed vessels = ampuls.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 AMPULS mean?
AMPULS noun- A small hermetically sealed vial, often used to contain a sterile solution suitable for injection.
- An Ancient Roman two-handled vessel.
- A vessel for containing consecrated wine or oil.
- The dilated end of a duct.
- The spongiole of a root.