The most common and most likely answer to this crossword clue is the 7 letter word NEBULAE
I've seen this crossword clue in Financial Times
N
E
B
U
L
A
E
More clues leading to the result NEBULAE
- Clouds having been formed containing radioactive element, note
- Luminous patches unable to be dispersed, absorbing energy
- Clouds from high blue area gathered in north-east
- Clouds unable to move disperse finally
- Gas clouds, unable to be dispersed, rose at last
- Astronomical features unable to be resolved heading east
- Clouds of blue an explosion initially sent out
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 = nebulae.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 NEBULAE mean?
NEBULAE noun- A cloud in outer space consisting of gas or dust (e.g. a cloud formed after a star explodes).
- A white spot or slight opacity of the cornea.
- A cloudy appearance in the urine