Top copy with illiterate's signature? (4)

I believe the answer is:

apex

I'm a little stuck... Click here to teach me more about this clue!

But I can't explain it at all! Could you help me understand?

(Other definitions for apex that I've seen before include "The highest point or peak" , "The highest point or pointed end" , "The highest point of something" , "Apogee, summit" , "Acme" .)

I've seen this clue in The Sun.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.