Schoolbook (6)
I believe the answer is:
primer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">schoolbook</span>' is the definition.<br/ ><span class="explanation-format__explanation">(primer is a kind of schoolbook)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for primer that I've seen before include "He charges" , "Solitary" , "introduction" , "Sealant coat" , "First school book" .)