Earls running from light (5)
I believe the answer is:
laser
'light' is the definition.
(lasers are a type of light)
'earls running' is the wordplay.
'running' is an anagram indicator (the letters run around to different places).
'earls' anagrammed gives 'LASER'.
'from' acts as a link.
(Other definitions for laser that I've seen before include "Device for concentrating light" , "Intense and concentrated beam of light" , "Device that produces an intense beam of light" , "Coherent light-beam" , "Powerful light source" .)