Heavy downpour (9)
(Other definitions for cloudburst that I've seen before include "dramatic downfall" , "Sudden rainstorm" , "Sudden, heavy downpour" , "sudden shower" , "storm" .)
'heavy downpour' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rainstorm that I've seen before include "Meteorological phenomenon" , "bad weather" , "power shower?" , "Wild weather" , "squall" .)