Sartre turned out to be most unusual (6)
I believe the answer is:
rarest
'be most unusual' is the definition.
(I've seen this before)
'sartre turned out' is the wordplay.
'turned out' indicates an anagram.
'sartre' with letters rearranged gives 'RAREST'.
'to' is the link.
(Other definitions for rarest that I've seen before include "Sartre (anag) -- most uncommon" , "fine to maximum" , "Most scarce or uncommon" , "Least common, most unusual" , "Most scarce or infrequent" .)