Real problem with small beam (5)
I believe the answer is:
laser
'beam' is the definition.
(laser beam is a kind of beam)
'real problem with small' is the wordplay.
'problem with' is an anagram indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'real' with its letters anagrammed with 's' makes 'LASER'.
(Other definitions for laser that I've seen before include "type of surgery" , "Intense light generator" , "Beam (used in surgery)" , "Device for concentrating light" , "Coherent light beam" .)