An opening in optics for a learner (5)
I believe the answer is:
pupil
This clue could be a double definition.
'an opening in optics' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'learner' is the second definition.
(pupil is a kind of learner)
'for a' is the link.
Can you help me to learn more?
(Other definitions for pupil that I've seen before include "Learner at educational institution" , "A student at school or eye part" , "One under instruction" , "Schoolchild; part of eye" , "one may see through!" .)