Extreme cold getting into kind of tyre (7)
I believe the answer is:
radical
'extreme' is the definition.
(thesaurus)
'cold getting into kind of tyre' is the wordplay.
'cold' becomes 'c' (eg on taps).
'getting into' indicates putting letters inside.
'kind of tyre' becomes 'radial' (I've seen this before).
'c' placed inside 'radial' is 'RADICAL'.
(Other definitions for radical that I've seen before include "Of reform, fundamental and innovative" , "Going to the root" , "Fundamental - thoroughgoing" , "Drastic and thorough, as reforms may be" , "Holding extreme political views" .)