Deeply recessed or hollow (6)
I believe the answer is:
sunken
I believe this is a double definition.
'deeply recessed' is the first definition.
(I've seen this before)
'hollow' is the second definition.
(similar in meaning)
(Other definitions for sunken that I've seen before include "Hollow" , "Bas relief" , "deeper level" , "Deeply recessed" , "One ac" .)