Preamble (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">preamble</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 intro that I've seen before include "Opening bars (abbr)" , "Instrumental part at start of song" , "Beginning (informal)" , "First bars of a musical composition" , "Start of work" .)