Beam at dancing earls (5)
I believe the answer is:
laser
'beam' is the definition.
(laser beam is a kind of beam)
'dancing earls' is the wordplay.
'dancing' indicates an anagram.
'earls' anagrammed gives 'LASER'.
'at' acts as a link.
(Other definitions for laser that I've seen before include "Device that generates an intense ray of light" , "Coherent light-beam" , "type of surgery" , "Powerful light source" , "Light-gun" .)