Reformist is thoroughgoing (7)
I believe the answer is:
radical
This clue is a double definition.
'reformist' is the first definition.
(thesaurus)
'thoroughgoing' is the second definition.
(thesaurus)
'is' is the link.
(Other definitions for radical that I've seen before include "Extreme, ultra" , "Tending towards extreme political views" , "Going to the root" , "Drastic and thorough, as reforms may be" , "stem" .)