The air with which men get out of line (4)
I believe the answer is:
mien
'the air' is the definition.
(I've seen this in another clue)
'men get out of line' is the wordplay.
'get' indicates putting letters inside.
'out of line' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
'men' placed around 'i' is 'MIEN'.
'with which' is the link.
Can you help me to learn more?
(Other definitions for mien that I've seen before include "The carriage" , "A person's demeanour" , "person's look" , "A face like mine" , "Comportment" .)