A singular aid to vision (8)
I believe the answer is:
eyeglass
This could be a double definition.
'a singular' is the first definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'aid to vision' is the second definition.
(I've seen this before)
Can you help me to learn more?
(Other definitions for eyeglass that I've seen before include "Lens for aiding defective vision" , "Monocle, or spectacles in plural" , "Aid to vision" .)