The most common and most likely answer to this crossword clue is the 5 plus 6 plus 4 letter word CLOUD CUCKOO LAND
I've seen this crossword clue in Financial Times
C
L
O
U
D
C
U
C
K
O
O
L
A
N
D
More clues leading to the result CLOUD CUCKOO LAND
Buddy explains!
The straight part
We know that "dream world" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dream world = cloud cuckoo land.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 CLOUD CUCKOO LAND mean?
CLOUD CUCKOO LAND noun- An imaginary place where silly or unrealistic people metaphorically reside.