Partially downgrades car tested by mathematician (9)
I believe the answer is:
descartes
'mathematician' is the definition.
(Descartes is an example)
'partially downgrades car tested' is the wordplay.
'partially' indicates the answer is hidden within the clue.
'DESCARTES' is hidden in the letters of 'downgrades car tested'.
'by' is the link.
(Other definitions for descartes that I've seen before include "Cogito, ergo sum man" , "One who thought" , "Rene --, philosopher" , "Cogito ergo sum philosopher" , "French mathematician and philosopher" .)