Knowledge largely hard to find by day? None of us can see it (8)
I believe the answer is:
infrared
'none of us can see it' is the definition.
I can't judge whether this defines the answer.
'knowledge largely hard to find by day?' is the wordplay.
'knowledge' becomes 'info' (short for information).
'largely' means to remove the last letter (most but not all of the letters).
'hard to find' becomes 'rare' (I've seen this in another clue).
'by' is a charade indicator (letters next to each other).
'day?' becomes 'd' (abbreviation for day).
'info' with its final letter taken off is 'inf'.
'inf'+'rare'+'d'='INFRARED'
Can you help me to learn more?
(Other definitions for infrared that I've seen before include "This ray enables one to see in the dark" , "Radiation from heated objects" , "(Of radiation) with a longer wavelength than light" , "Fan drier (anag) - type of radiation" , "sort of photography" .)