Introductory section (8)
I believe the answer is:
prologue
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">introductory section</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 prologue that I've seen before include "Introduction to a play or story" , "Start to play" , "Introductory scene" , "Introductory passage" , "Introductory speech" .)