Extremist brutal having released new form of radiation (11)
I believe the answer is:
ultraviolet
'form of radiation' is the definition.
(I know that ultraviolet is a type of actinic radiation)
'extremist brutal' is the wordplay.
'extremist' becomes 'ultra' (ultra can mean an extremist or radical).
'brutal' becomes 'violet' (I can't justify this - if you can you should believe this answer much more).
'ultra'+'violet'='ULTRAVIOLET'
'having released new' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ultraviolet that I've seen before include "Radiation just outside the visible spectrum" , "light range" , "electromagnetic radiation" , "Relating to part of non-visible spectrum" , "form of invisible radiation" .)