The most common and most likely answer to this crossword clue is the 6 plus 2 plus 3 plus 3 letter word CASTLE IN THE AIR
I've seen this crossword clue in The Guardian
C
A
S
T
L
E
I
N
T
H
E
A
I
R
More clues leading to the result CASTLE IN THE AIR
Buddy explains!
The straight part
We know that "Dream/rook in flight" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dream/rook in flight = castle in the air.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.