Introductory textbook (6)
I believe the answer is:
primer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">introductory textbook</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 primer that I've seen before include "Educational book" , "Preparatory coat of paint" , "Basic book; undercoat" , "instructive book" , "explosive device" .)