Signalling apparatus (9)
I believe the answer is:
semaphore
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">signalling apparatus</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 semaphore that I've seen before include "Messaging by flags" , "Signalling apparatus" , "Way to signal" , "flagging" , "Old flag signalling system" .)