River valley starts to widen after downpour initially (4)
I believe the answer is:
wadi
'river valley' is the definition.
'starts to widen after downpour initially' is the wordplay.
'starts to' says to take the initial letters.
The initial letters of 'widen after downpour initially' is 'WADI'.
(Other definitions for wadi that I've seen before include "Dry North African river bed" , "North African gully, dry except in the rainy season" , "Seasonal water channel" , "Seasonally dry channel" , "Mainly dry ravine" .)