Broadcasting medium (5)
I believe the answer is:
radio
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">broadcasting medium</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for radio that I've seen before include "MW, LW or FM sound source" , "Broadcast receiver" , "It was first called the wireless" , "send a message" , "Receiving device" .)