Sectarian rambling of a philosopher (9)
I believe the answer is:
cartesian
'philosopher' is the definition.
(thesaurus)
'sectarian rambling' is the wordplay.
'rambling' indicates an anagram.
'sectarian' anagrammed gives 'CARTESIAN'.
'of a' acts as a link.
(Other definitions for cartesian that I've seen before include "Relating to a particular French philosopher" , "Of Descartes or his theories - maybe sectarian" , "Following a particular philosopher" , "Pertaining to a particular French philosopher" , "Philosopher's follower" .)