Underlying reason (9)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">underlying reason</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for rationale that I've seen before include "Logic, reasoning" , "explanation" , "grounds?" , "Underlying principle" , "Why regulate" .)
(Other definitions for root cause that I've seen before include "Fundamental reason" , "The origin of a problem" .)