Deeply recessed or hollow (6)
I believe the answer is:
sunken
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">deeply recessed</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">hollow</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ ></span>
(Other definitions for sunken that I've seen before include "Hollow" , "Bas relief" , "deeper level" , "Deeply recessed" , "One ac" .)