Extremist spoke about onset of change (7)
I believe the answer is:
radical
'extremist' is the definition.
('radical' can be a synonym of 'extremist')
'spoke about onset of change' is the wordplay.
'spoke' becomes 'radial' (I've seen this before).
'about' means one lot of letters goes inside another.
'onset of' suggests taking the first letters.
The first letter of 'change' is 'c'.
'radial' going around 'c' is 'RADICAL'.
(Other definitions for radical that I've seen before include "Innovative, progressive" , "Primitive" , "(One) favouring fundamental changes" , "Progressive; extreme" , "Sweeping - reformer" .)