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It's for those who need fresh air in the main (8)

Ross

I believe the answer is:

aqualung

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'main' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for aqualung that I've seen before include "Diver's breathing apparatus" , "Underwater breathing apparatus" , "Form of scuba" , "Diving equipment with cylinders of compressed air" , "Aid for diver" .)

I've seen this clue in The Telegraph.
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