Clara, upset about Di being extremist (7)
I believe the answer is:
radical
'being extremist' is the definition.
('radical' can be a synonym of 'extremist')
'clara upset about di' is the wordplay.
'upset' indicates anagramming the letters.
'about' is an insertion indicator.
'clara' is an anagram of 'racal'.
'racal' enclosing 'di' is 'RADICAL'.
(Other definitions for radical that I've seen before include "Drastic or thorough as changes may be" , "One who favours thoroughgoing reform" , "Innovative, progressive" , "Extreme, ultra" , "Holding extreme political views" .)