Hill of sand (4)
I believe the answer is:
dune
'hill 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 "Mound of drifted sand" , "Nude in the sand" , "Sandhill" , "Sandy hill" , "Low hill of sand, called in a desert a barkhan" .)