Nitrogen, say, is about to happen (2,3,3)
I believe the answer is:
in the air
'about to happen' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for in the air that I've seen before include "Unsettled" , "How one might describe song's contents" , "Current, exerting an influence - may be floating" , "Undecided; current" , "Prevalent in a vague form" .)