Sand ridge (4)
I believe the answer is:
dune
'sand ridge' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dune that I've seen before include "Sandy hill" , "Ridge of wind-blown sand" , "Low hill of sand, called in a desert a barkhan" , "Seashore sand hill" , "Mound of drifted sand" .)