The most common and most likely answer to this crossword clue is the 8 letter word AMPHORAE
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The straight part
We know that "jars" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jars = amphorae.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 AMPHORAE mean?
AMPHORAE noun- A two-handled jar with a narrow neck that was used in ancient times to store or carry wine or oil.
- One of various units for measuring liquid or volume during the Roman Empire, measuring between 18.5 and 39 litres depending on the variant.
- Ancient unit of volume, for the measurement of the internal capacity of a ship.
- In botany, the lower valve of the fruit that opens transversely.