Air alternative (4)
I believe the answer is:
rail
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">air alternative</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)