Fundamental; extreme (7)
I believe the answer is:
radical
This clue is a double definition.
'fundamental' 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 "Politically extreme" , "stem" , "Person with extreme political views" , "Deep-seated" , "One with rooted" .)