Hill of sand (4)
I believe the answer is:
dune
'hill of sand' is the definition.
(I've seen this in another clue)
This is the entire 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" .)