Extremist spoke about start of communism (7)
I believe the answer is:
radical
'extremist' is the definition.
('radical' can be a synonym of 'extremist')
'spoke about start of communism' is the wordplay.
'spoke' becomes 'radial' (I've seen this before).
'about' is an insertion indicator.
'start of' suggests taking the first letters.
The first letter of 'communism' is 'c'.
'radial' going around 'c' is 'RADICAL'.
(Other definitions for radical that I've seen before include "Primitive" , "One with rooted" , "Political extremist" , "Holding extreme political views" , "Dial car in order to be drastic" .)