The most common and most likely answer to this crossword clue is the 6 letter word SCUBAS
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The straight part
We know that "self-contained underwater breathing apparatuses" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case self-contained underwater breathing apparatuses = scubas.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 SCUBAS mean?
SCUBAS noun- (underwater diving) An apparatus carried by a diver, which includes a tank holding compressed, filtered air and a regulator which delivers the air to the diver at ambient pressure which can be used underwater.