The most common and most likely answer to this crossword clue is the 9 letter word PNEUMONIA
P
N
E
U
M
O
N
I
A
More clues leading to the result PNEUMONIA
- Menu distributed during piano playing gives rise tocomplaint
- Unfinished pie on a menu sadly leading to complaint
- Condition of coasts ignored by uncompassionate criminal
- Knew more complaining reportedly causes serious illness
- Geordie wrestler cutting head, in pain, developed illness
- Recent lament aired by setter: a problem with inspiration
- New German doctor blocks pain to treat illness
- MD engaging with this disease unsuccessfully, resulting in pandemonium
- Fresh complaint here is said to be a serious illness
- Illness being new, complaining more to listeners
Buddy explains!

The straight part
We know that "illness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case illness = pneumonia.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 PNEUMONIA mean?
PNEUMONIA noun- An acute or chronic inflammation of the lungs caused by viruses, bacteria or other microorganisms, or sometimes by physical or chemical irritants.