The most common and most likely answer to this crossword clue is the 13 letter word STRATOCUMULUS
I've seen this crossword clue in The Times
S
T
R
A
T
O
C
U
M
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L
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S
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The straight part
We know that "cloud" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cloud = stratocumulus.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 STRATOCUMULUS mean?
STRATOCUMULUS noun- A principal low-level cloud type, predominantly stratiform, in the form of a gray and/or whitish layer or patch, which nearly always has dark parts and is nonfibrous.