One who doubts accepted opinions (7)
I believe the answer is:
sceptic
'one who doubts accepted opinions' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sceptic that I've seen before include "One is inclined to doubt accepted opinions" , "Disbeliever" , "Greek philosopher" , "One who doubts accepted opinions" , "Thomas for one" .)