Light beam (5)
I believe the answer is:
laser
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">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" , "Generator of beam" , "Device yielding powerful beam" , "Device producing an intense beam of light or radiation" , "type of surgery" .)