Clever but fallacious reasoner (7)
I believe the answer is:
sophist
'clever but fallacious reasoner' is the definition.
(I've seen this before)
This is all the clue.
(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" .)