Sink into natural depression (5)
I believe the answer is:
basin
I believe this is a double definition.
'sink' is the first definition.
(as in a wash basin)
'natural depression' is the second definition.
(basin is a kind of natural depression)
'into' is the link.
(Other definitions for basin that I've seen before include "Stoup" , "Water receptacle" , "dock, maybe" , "Large bowl" , "River-drained area" .)