The most common and most likely answer to this crossword clue is the 6 letter word CUMULI
I've seen this crossword clue in The Guardian
C
U
M
U
L
I
More clues leading to the result CUMULI
Buddy explains!

The straight part
We know that "clouds" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case clouds = cumuli.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 CUMULI mean?
CUMULI noun- A large white puffy cloud that develops through convection. On a hot, humid day, they can form towers and even become cumulonimbus clouds.
- A mound or heap.