The most common and most likely answer to this crossword clue is the 12 letter word NIMBOSTRATUS
I've seen this crossword clue in The Daily Telegraph
N
I
M
B
O
S
T
R
A
T
U
S
Buddy explains!
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 = nimbostratus.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 NIMBOSTRATUS mean?
NIMBOSTRATUS noun- According to the World Meteorological Organization, a mid-level, principal cloud type, generally formless and dark grey in colour, which forms from altostratus occurring in layers at the middle altitude of the troposphere (usually above 2400 metres). Nimbostratus usually brings precipitation as the mid-level clouds thicken and subside into the low level of the troposphere. Frontal or cyclonic lift can also carry the top of a deep nimbostratus layer into the high levels of the troposphere. Also classified or characterized as multi-level; abbreviated Ns.