Doubter, cynic (7)
I believe the answer is:
sceptic
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">doubter</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">cynic</span>' is the second definition.<br/ ><br/ ></span>
(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" .)