It puts one pupil under glass (7)
I believe the answer is:
monocle
'glass' is the definition.
(I've seen this before)
'it puts one pupil under' is the wordplay.
I cannot quite see how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is located in the answer.
'one' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'pupil' could be 'l' (as in a learner driver with L-plates on their car) and 'l' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for monocle that I've seen before include "Reading glass for one eye only" , "Single eyeglass" , "One-lens seeing apparatus" , "lens" , "optical device" .)