A proposed explanation (10)
I believe the answer is:
hypothesis
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a proposed explanation</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 hypothesis that I've seen before include "Guess" , "Speculation" , "Tentative proposition" , "conjecture" , "What's supposed" .)