Diogenes, for one (5)
I believe the answer is:
cynic
'diogenes for one' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cynic that I've seen before include "Misanthrope" , "Sort of Greek philosopher" , "Person who sneers at others, a sceptic" , "One who believes the worst of people" , "Sceptic; philosopher" .)