It's for those who need fresh air in the main (8)
I believe the answer is:
aqualung
'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" .)