Radical (9)
'radical' is the definition.
(I know that extremist is a type of radical)
This is all the clue.
(Other definitions for extremist that I've seen before include "Revolutionary" , "One far from moderate" , "One with highly non-standard views" , "immoderate person" , "One with views far from the mainstream" .)
'radical' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for ultra that I've seen before include "holding extreme position" , "Prefix meaning extreme or excessive" , "Secret intelligence" , "lacking moderation" , "(One who is) extreme (in eg political opinion)" .)