Look in one day, and you'll find him in
I believe the answer is:
ned
'you'll find him in' is the definition.
The definition suggests a singular noun which matches the answer.
'look in one day' is the wordplay.
'look in one' becomes 'ne' (I am not sure about this - if you are sure you should believe this answer much more).
'day' becomes 'd' (abbreviation for day).
'ne'+'d'='NED'
'and' is the link.
Can you help me to learn more?
(Other definitions for ned that I've seen before include "rough" , "A den for Edward" .)