Under discussion, like many factory emissions? (2,3,3)
I believe the answer is:
in the air
'under' is the definition.
Both the answer and definition are adverbs. Perhaps you can see a link between them that I can't see?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for in the air that I've seen before include "About to happen" , "Where clouds are" , "Prevalent in a vague form" , "Unsettled" , "when travelling by plane" .)