The most common and most likely answer to this crossword clue is the 12 letter word MALEVOLENTLY
I've seen this crossword clue in The Guardian
M
A
L
E
V
O
L
E
N
T
L
Y
Buddy explains!
The straight part
We know that "with ill will" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case with ill will = malevolently.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.