The most common and most likely answer to this crossword clue is the 7 letter word GRAPHIC
I've seen this crossword clue in The Daily Telegraph
G
R
A
P
H
I
C
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 = graphic.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 GRAPHIC mean?
GRAPHIC noun- A drawing or picture.
- (mostly in plural) A computer-generated image as viewed on a screen forming part of a game or a film etc.
- Drawn, pictorial.
- Vivid, descriptive, often in relation to depictions of sex or violence.
- Having a texture that resembles writing, commonly created by exsolution, devitrification and immiscibility processes in igneous rocks.