Light beam that's very strong (5)
I believe the answer is:
'light beam' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for laser that I've seen before include "Coherent light-beam" , "Device producing thin beam of light" , "Device that generates an intense beam of light" , "E.g. surgical light-beam" , "Beam (used in surgery)" .)