Map-book (5)
I believe the answer is:
atlas
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">map</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">book</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(atlas is a kind of book)</span><br/ ><br/ ></span>
(Other definitions for atlas that I've seen before include "One hugely burdened" , "Titan who supported the heavens" , "N. African mountains; a Titan" , "Legendary weightlifter" , "Map collection" .)