Do up corner nearly all in the dark
I believe the answer is:
nocturnal
'dark' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I can't see?
'do up corner nearly all' is the wordplay.
'do' becomes 'con' (both can mean to defraud).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'corner' becomes 'turn' (I can't justify this - if you can you should give a lot more credence to this answer).
'nearly all' becomes 'al'.
'con' in reverse letter order is 'noc'.
'noc'+'turn'+'al'='NOCTURNAL'
'in the' acts as a link.
Can you help me to learn more?
(Other definitions for nocturnal that I've seen before include "shunning the light" , "Appertaining to the night" , "Like bats" , "occurring at night" , "Active at night" .)