Earls involved in intense concentration (5)
I believe the answer is:
laser
'intense concentration' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'earls involved' is the wordplay.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'earls' anagrammed gives 'LASER'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for laser that I've seen before include "Device that generates an intense ray of light" , "light-intensifier" , "Coherent light-beam" , "scientific instrument" , "Coherent light beam" .)