Deceptive (8)
(Other definitions for specious that I've seen before include "Superficially plausible but actually wrong" , "Copies us, being superficially good" , "Apparently correct, but actually wrong" , "Plausibly deceptive" , "Superficially sound but without real merit" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">deceptive</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 illusory that I've seen before include "in the imagination" , "Deceptive" , "Unreal, fanciful" , "Fanciful, imaginary (8)" , "false" .)