Coherent light beam (5)
I believe the answer is:
laser
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">coherent light beam</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for laser that I've seen before include "Surgical device" , "Powerful light source" , "Intense light generator" , "Device producing thin beam of light" , "Source of intense beam" .)