The most common and most likely answer to this crossword clue is the 14 letter word BROKEN-HEARTED
I've seen this crossword clue in Financial Times
B
R
O
K
E
N
-
H
E
A
R
T
E
D
Buddy explains!
The straight part
We know that "Bitterly disappointed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bitterly disappointed = broken-hearted.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 BROKEN-HEARTED mean?
BROKEN-HEARTED adjective- Grieved and disappointed, especially with the loss of a beloved person or thing, such as the repudiation of a romantic relationship.