Militant spoke about Conservative (7)
I believe the answer is:
radical
'militant' is the definition.
(thesaurus)
'spoke about conservative' is the wordplay.
'spoke' becomes 'radial' (I've seen this before).
'about' is an insertion indicator.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'radial' enclosing 'c' is 'RADICAL'.
(Other definitions for radical that I've seen before include "Thoroughgoing - (popularly) extremist" , "Primitive" , "Tending towards extreme political views" , "One seeking root and branch reform" , "Drastic or thorough as changes may be" .)