Rarest sort of philosopher (6)
I believe the answer is:
sartre
'philosopher' is the definition.
(I've seen this before)
'rarest sort' is the wordplay.
'sort' indicates an anagram.
'rarest' is an anagram of 'SARTRE'.
'of' is the link.
(Other definitions for sartre that I've seen before include "Jean-Paul --, French author and philosopher" , "Jean-Paul -, French philosopher/author" , "French philosopher, 1905-80" , "Arrest (anag) - French philosopher, d. 1980" , "Jean Paul . . ., French existentialist writer" .)