The most common and most likely answer to this crossword clue is the 12 letter word UNACCEPTABLE
U
N
A
C
C
E
P
T
A
B
L
E
Buddy explains!

The straight part
We know that "Disappointing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case disappointing = unacceptable.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 UNACCEPTABLE mean?
UNACCEPTABLE noun- Something that is not acceptable.
- Unsatisfactory; not acceptable
- Not conforming to accepted usage