Clever but fallacious reasoner (7)
I believe the answer is:
sophist
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">clever but fallacious reasoner</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for sophist that I've seen before include "One given to specious reasoning" , "One who uses clever but false argument" , "One using clever but wrong arguments" , "One using clever but fallacious arguments" , "one might plausibly argue" .)