Crossword Genius

One to observe pupils very closely (8)

Ross

I believe the answer is:

optician

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

'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for optician that I've seen before include "where sharper look sold" , "Maker or seller of spectacles" , "Expert helping people see" , "Prescriber of glasses and contact lenses" , "I make visual inspections" .)

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.