Doubter, cynic (7)
I believe the answer is:
sceptic
This clue is a double definition.
'doubter' is the first definition.
(synonyms)
'cynic' is the second definition.
(Other definitions for sceptic that I've seen before include "Disbeliever" , "Agnostic" , "One who doubts accepted opinions" , "Thomas for one" , "One habitually doubts accepted beliefs" .)