It's smart to go by air (3)
I believe the answer is:
fly
I believe this is a double definition.
'it's smart' is the first definition.
(fly can informally mean smart or well-dressed)
'go by air' is the second definition.
(I've seen this in another clue)
'to' acts as a link.
(Other definitions for fly that I've seen before include "Glide" , "Travel through the air" , "Insect; go quickly" , "Run away; smart (colloq.)" , "SECOND PART OF 14" .)