Visual aid designed for single pupil (7)
I believe the answer is:
monocle
'visual aid' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?
'designed for single pupil' is the wordplay.
I cannot really see how this works, but
'single' could be 'mono' (prefix meaning single) and 'mono' is present in the answer.
'pupil' could be 'l' (as in a learner driver with L-plates on their car) and 'l' is found within the answer.
This explanation may well be incorrect...
(Other definitions for monocle that I've seen before include "Single eyeglass" , "Reading glass for one eye only" , "One-lens seeing apparatus" , "optical device" , "lens" .)