Radical mix setter concocted (9)
I believe the answer is:
extremist
'radical' is the definition.
(I know that extremist is a type of radical)
'mix setter concocted' is the wordplay.
'concocted' is an anagram indicator.
'mix'+'setter'='mixsetter'
'mixsetter' anagrammed gives 'EXTREMIST'.
(Other definitions for extremist that I've seen before include "immoderate person" , "An ultra-radical" , "One with highly non-standard views" , "Revolutionary" , "One far from moderate" .)