The most common and most likely answer to this crossword clue is the 7 letter word LEPROSY
L
E
P
R
O
S
Y
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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 = leprosy.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 LEPROSY mean?
LEPROSY noun- An infectious disease caused by infection by Mycobacterium leprae.
- In the Bible, a disease of the skin not conclusively identified, which can also affect clothes and houses.