The most common and most likely answer to this crossword clue is the 9 plus 6 letter word PLANETARY NEBULA
I've seen this crossword clue in The Guardian
P
L
A
N
E
T
A
R
Y
N
E
B
U
L
A
Buddy explains!
The straight part
We know that "luminous cloud" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case luminous cloud = planetary nebula.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.