Concave (6)
I believe the answer is:
hollow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">concave</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for hollow that I've seen before include "Dent; echoey" , "Insincere; not solid" , "Not solid, having hole in the middle" , "Empty, not solid" , "Empty - insincere" .)