It dawns on most people in bed (8)
I believe the answer is:
daybreak
'it dawns' is the definition.
The definition suggests a singular noun which matches the answer.
'on most people in bed' is the wordplay.
I cannot quite understand how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for daybreak that I've seen before include "Time for larks" , "Cockcrow" , "When light first appears" , "Dawn" , "very short holiday" .)