The most common and most likely answer to this crossword clue is the 12 letter word DISENCHANTED
I've seen this crossword clue in The Guardian
D
I
S
E
N
C
H
A
N
T
E
D
More clues leading to the result DISENCHANTED
Buddy explains!

The straight part
We know that "Disappointed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case disappointed = disenchanted.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 DISENCHANTED mean?
DISENCHANTED verb- (of a person) To free from illusion, false belief or enchantment; to undeceive or disillusion.
- (of a person) To disappoint.
- (of a thing) To remove a spell or magic enchantment from.