Earls wandering in the light (5)
I believe the answer is:
laser
'light' is the definition.
(lasers are a type of light)
'earls wandering' is the wordplay.
'wandering' indicates an anagram.
'earls' anagrammed gives 'LASER'.
'in the' acts as a link.
(Other definitions for laser that I've seen before include "Light amplification by stimulated emission of radiation" , "type of surgery" , "scientific device" , "Source of intense beam" , "light-intensifier" .)