Signalling code (5)
I believe the answer is:
morse
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">signalling code</span>' is the definition.<br/ ><span class="explanation-format__explanation">(Morse code)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for morse that I've seen before include "& 24 Across Encrypted language" , "used to transmit messages" , "TV inspector" , "Code inventor, Samuel" , "Dot-dash code inventor" .)