Mound of sand (4)
I believe the answer is:
dune
'mound of sand' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dune that I've seen before include "Sandy hill" , "Mound of drifted sand" , "Ridge of wind-blown sand" , "Ridge of sand created by wind" , "Seashore sand hill" .)