Air alternative (4)
I believe the answer is:
rail
'air alternative' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rail that I've seen before include "Complain vehemently" , "Bar - marsh bird" , "Bird with long legs" , "The Iron Horse road" , "20 backwards, that 7 runs on" .)