Original sort of tyre round about (7)
I believe the answer is:
radical
'original' is the definition.
(thesaurus)
'sort of tyre round about' is the wordplay.
'sort of tyre' becomes 'radial' (I've seen this before).
'round' means one lot of letters goes inside another.
'about' becomes 'c' (abbreviation for 'circa'**).
'radial' enclosing 'c' is 'RADICAL'.
(Other definitions for radical that I've seen before include "One with rooted" , "Thoroughgoing - (popularly) extremist" , "left-winger" , "Political extremist" , "Sweeping - reformer" .)