Sand mound (4)
I believe the answer is:
dune
'sand mound' 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 "Seashore sand hill" , "Low hill of sand, called in a desert a barkhan" , "Ridge of wind-blown sand" , "Coastal sand hill" , "Sandy hill" .)