Sceptic (5)
I believe the answer is:
cynic
'sceptic' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cynic that I've seen before include "One who takes a pessimistic view of human goodness or sincerity" , "One who believes people's motives are always selfish" , "Sceptic; philosopher" , "Philosopher as Diogenes" , "Misanthrope" .)