Device producing an intense beam of light (5)
I believe the answer is:
laser
'device producing an intense beam of light' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for laser that I've seen before include "Device producing an intense light beam - kind of sailing dinghy" , "E.g. surgical light-beam" , "Beam (used in surgery)" , "Surgical device" , "scientific instrument" .)