Cause, justification (6)
I believe the answer is:
reason
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">cause</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">justification</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for reason that I've seen before include "Employ logic" , "Explanation; logic" , "Justification (for action)" , "Reach logical conclusions" , "Cause; think logically" .)