The most common and most likely answer to this crossword clue is the 10 letter word OFF-COLOUR
O
F
F
-
C
O
L
O
U
R
More clues leading to the result OFF-COLOUR
Buddy explains!

The straight part
We know that "ill" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ill = off-colour.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 OFF-COLOUR mean?
OFF-COLOUR adjective- Of the wrong color.
- Dirty, vulgar or obscene, especially in a way that is unpleasant.
- Different than usual.
- Feeling mildly unwell.