Elementary textbook (6)
I believe the answer is:
primer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">elementary textbook</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for primer that I've seen before include "explosive device" , "Introductory textbook or first coat of paint" , "Preparatory coat of paint" , "Basic book; undercoat" , "First coat - book" .)