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