Sign at a radio station (2,3)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sign at a radio station</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for on air that I've seen before include "very happily?" , "In the act of broadcasting" , "In the process of broadcasting on radio or TV" , "How one walks when elated" , "now showing" .)