Abstract idea (7)
I believe the answer is:
concept
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">abstract idea</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for concept that I've seen before include "Abstract idea; mental picture" , "Product of the mind" , "Abstract or general idea" , "Notion" .)