Not solid (6)
I believe the answer is:
hollow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not solid</span>' is the definition.<br/ ><span class="explanation-format__explanation">('hollow' is the opposite to 'solid')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for hollow that I've seen before include "Empty in the middle" , "Empty; worthless" , "Cavity or space in something" , "Insincere; not solid" , "Empty space within" .)