The most common and most likely answer to this crossword clue is the 7 letter word CAISSON
I've seen this crossword clue in The Guardian
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Buddy explains!
The straight part
We know that "Watertight chamber" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case watertight chamber = caisson.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 CAISSON mean?
CAISSON noun- An enclosure from which water can be expelled, in order to give access to underwater areas for engineering works etc.
- The gate across the entrance to a dry dock.
- A floating tank that can be submerged, attached to an underwater object and then pumped out to lift the object by buoyancy; a camel.
- A two-wheeled, horse-drawn military vehicle used to carry ammunition (and a coffin at funerals).
- A large box to hold ammunition.
- A chest filled with explosive materials, used like a mine.
- A coffer.