The most common and most likely answer to this crossword clue is the 12 letter word FRONTISPIECE
I've seen this crossword clue in The Daily Telegraph
F
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More clues leading to the result FRONTISPIECE
Buddy explains!

The straight part
We know that "Illustration" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case illustration = frontispiece.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 FRONTISPIECE mean?
FRONTISPIECE noun- An illustration that is on the page before the title page of a book, a section of one, or a magazine.
- The title page of a book.
- A façade, especially an ornamental one.
- A small ornamental pediment, especially at the top of a window or door.
- To supply with a frontispiece.