Extremist spoke about capitalism, primarily (7)
I believe the answer is:
radical
'extremist' is the definition.
(I know that extremist can be written as radical)
'spoke about capitalism primarily' is the wordplay.
'spoke' becomes 'radial' (I've seen this before).
'about' means one lot of letters goes inside another.
'primarily' says to take the initial letters.
The first letter of 'capitalism' is 'c'.
'radial' going around 'c' is 'RADICAL'.
(Other definitions for radical that I've seen before include "(One) favouring fundamental changes" , "stem" , "Progressive - car dial (anag)" , "Primitive" , "Extreme, ultra" .)