Textbook (6)
I believe the answer is:
primer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">textbook</span>' is the definition.<br/ ><span class="explanation-format__explanation">(primer is a kind of textbook)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for primer that I've seen before include "Preparatory coat of paint" , "Elementary text book" , "He charges" , "Basic teaching book" , "Sealant coat" .)