The most common and most likely answer to this crossword clue is the 12 letter word THUNDERSTORM
I've seen this crossword clue in The Guardian
T
H
U
N
D
E
R
S
T
O
R
M
More clues leading to the result THUNDERSTORM
Buddy explains!
The straight part
We know that "cloudburst" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cloudburst = thunderstorm.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does THUNDERSTORM mean?
THUNDERSTORM noun- A storm consisting of thunder and lightning produced by a cumulonimbus, usually accompanied with heavy rain, wind, and sometimes hail; and in rarer cases sleet, freezing rain, or snow.