The most common and most likely answer to this crossword clue is the 10 letter word AIRY-FAIRY
I've seen this crossword clue in Financial Times
A
I
R
Y
-
F
A
I
R
Y
More clues leading to the result AIRY-FAIRY
Buddy explains!

The straight part
We know that "unrealistic" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unrealistic = airy-fairy.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 AIRY-FAIRY mean?
AIRY-FAIRY adjective- Insubstantial
- Impractical