Powerful beam (5)
I believe the answer is:
laser
'powerful beam' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for laser that I've seen before include "type of surgery" , "Device that produces an intense beam of light" , "E.g. surgical light-beam" , "scientific instrument" , "Intense and concentrated beam of light" .)