Solid, three-dimensional (5)
I believe the answer is:
cubic
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">solid</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">three-dimensional</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that three-dimensional can be written as cubic)</span><br/ ><br/ ></span>
(Other definitions for cubic that I've seen before include "Shaped like a solid square" , "With six equal faces" , "puzzling sort of equation" , "Having three dimensions" , "Shaped like a die" .)