Exercise book (6)
I believe the answer is:
primer
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">exercise book</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for primer that I've seen before include "Preparatory coat of paint" , "Introductory textbook or first coat of paint" , "introductory book" , "Solitary" , "Basic teaching book" .)