Valid reasoning (5)
I believe the answer is:
logic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">valid reasoning</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for logic that I've seen before include "System of reasoning" , "Rigorous reasoning" , "this makes sense" , "4-ing" , "Sound reasoning" .)