Dry riverbed (4)
I believe the answer is:
wadi
'dry riverbed' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for wadi that I've seen before include "Seasonal water course (in Arabic-speaking lands)" , "dry course" , "North African gully, dry except in the rainy season" , "Rocky watercourse in N Africa" , "(Usually) dry desert riverbed" .)