At a lower level than the surrounding area (6)
I believe the answer is:
sunken
'at a lower level than the surrounding area' is the definition.
(I've seen this before)
This is all the clue.
(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" .)