The most common and most likely answer to this crossword clue is the 5 letter word MOONY
I've seen this crossword clue in The Daily Telegraph
M
O
O
N
Y
Buddy explains!

The straight part
We know that "Dreamy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dreamy = moony.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 MOONY mean?
MOONY noun- The act of mooning, flashing the buttocks.
- A silly person.
- Resembling the moon.
- Moonlit.
- Absent-minded.
- Silly.
- Sickly or tipsy.