Natural depression (4)
I believe the answer is:
dell
'natural depression' is the definition.
(dell is a kind of natural depression)
This is all the clue.
(Other definitions for dell that I've seen before include "Small wooded hollow or valley" , "Deep hollow or small valley" , "Small valley; computer company" , "Small valley, rhymes with tumbled down" , "Small secluded valley" .)