Dry watercourse (4)
I believe the answer is:
wadi
'dry watercourse' 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 dry watercourse" , "dry course" , "Rocky watercourse in N Africa" , "Seasonal water channel" , "Valley usually dry" .)