Real trouble with small beam (5)
I believe the answer is:
laser
'beam' is the definition.
(laser beam is a kind of beam)
'real trouble with small' is the wordplay.
'trouble with' is an anagram indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'real' mixed up with 's' can make 'LASER'.
(Other definitions for laser that I've seen before include "Powerful light source" , "Light-gun" , "Intense and concentrated beam of light" , "Intense light generator" , "Device that generates an intense beam of light" .)