Radical mix setter's deployed (9)
I believe the answer is:
extremist
'radical' is the definition.
(I know that radical can be written as extremist)
'mix setter's deployed' is the wordplay.
'deployed' indicates an anagram.
'mix'+'setter'='mixsetter'
'mixsetter' with letters rearranged gives 'EXTREMIST'.
(Other definitions for extremist that I've seen before include "Fanatic, hardliner" , "One with highly non-standard views" , "Revolutionary" , "His views are anything but neutral" , "immoderate person" .)