Spectator's using toilet before King and Queen arrive (8)

I believe the answer is:

onlooker

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

'spectator's' is the definition.
(onlooker is a kind of spectator)

I cannot understand how the rest of the clue works.

(Other definitions for onlooker that I've seen before include "One who watches without becoming involved" , "One observes" , "Non-participant" , "Spectator" , "Person watching" .)

I've seen this clue in The Guardian.

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.