At a lower level than the surrounding area (6)
I believe the answer is:
sunken
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">at a lower level than the surrounding area</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for sunken that I've seen before include "At a lower level than the surrounding area" , "deeper level" , "(Of eyes) recessed" , "Deeply recessed" , "One ac" .)