Real worried about second beam (5)
I believe the answer is:
laser
'beam' is the definition.
(laser beam is a kind of beam)
'real worried about second' is the wordplay.
'worried' is an anagram indicator.
'about' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'real' anagrammed gives 'laer'.
'laer' enclosing 's' is 'LASER'.
(Other definitions for laser that I've seen before include "Device that generates an intense beam of light" , "light-intensifier" , "Coherent light source" , "Light amplification by stimulated emission of radiation" , "Beam (used in surgery)" .)