One to observe pupils very closely (8)
I believe the answer is:
optician
'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" .)