Thoroughgoing or extreme (7)
I believe the answer is:
radical
This clue is a double definition.
'thoroughgoing' is the first definition.
(I've seen this before)
'extreme' is the second definition.
(I've seen this in another clue)
(Other definitions for radical that I've seen before include "Drastic and thorough, as reforms may be" , "Basic" , "Fundamental - thoroughgoing" , "Sweeping - reformer" , "Fundamental - non-conformist" .)