Heavy rainfall (8)
'heavy rainfall' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for downpour that I've seen before include "Big fall makes" , "Deluge" , "Cloudburst" , "Heavy fall of rain" , "Heavy continuous rainfall" .)
(Other definitions for cloudburst that I've seen before include "dramatic downfall" , "Sudden, heavy downpour" , "sudden shower" , "Sudden rainstorm" , "storm" .)