Not hollow (5)
I believe the answer is:
solid
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not hollow</span>' is the definition.<br/ ><span class="explanation-format__explanation">('solid' is the opposite to 'hollow')</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for solid that I've seen before include "Firm - three-dimensional" , "Substantial - having three dimensions" , "Three-dimensional shape, not hollow" , "Reliable - substantial - pure" , "Reliable; not hollow" .)