School book (6)
I believe the answer is:
primer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">school book</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 "Elementary textbook" , "Sealant coat" , "First coat - book" , "Introductory textbook or first coat of paint" , "device for detonating charge" .)