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