One learning to let in the light? (5)
I believe the answer is:
pupil
This clue could be a double definition.
'one learning' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'let in the light?' is the second definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'to' acts as a link.
Can you help me to learn more?
(Other definitions for pupil that I've seen before include "Trainee barrister" , "Aperture in an eye" , "Schoolboy or eye part" , "Eye opening" , "that shrinks from brightness" .)