The most common and most likely answer to this crossword clue is the 4 plus 2 plus 1 plus 6 letter word SICK AS A PARROT
I've seen this crossword clue in The Times
S
I
C
K
A
S
A
P
A
R
R
O
T
More clues leading to the result SICK AS A PARROT
- Matching Polly’s disappointment?
- Really gutted to be suffering bird flu?
- Disappointed, apparently seeking veterinary assistance
- Extremely disappointed, soon to feature in famous comedy sketch?
- Demolished a car park – so it’s gutted
- In review, “Tosca lacks middle register ultimately, Scarpia’s OK” – so very disappointed
- Suffering food poisoning perhaps when a fowl is badly gutted
- Gutted, suffering psittacosis?
- Down with psittacosis?
- Down with the same bug as Polly?
Buddy explains!

The straight part
We know that "Extremely disappointed," is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely disappointed, = sick as a parrot.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.