Dry streambed (4)
I believe the answer is:
wadi
'dry streambed' 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 "Seasonally wet ravine" , "River valley; dry bed" , "seasonal flower" , "Normally dry ravine" , "North African gully, dry except in the rainy season" .)