At home, key outside ... it's dark (4)
I believe the answer is:
inky
'dark' is the definition.
(I've seen this in another clue)
'at home key outside' is the wordplay.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'outside' means to remove the middle letters.
'key' with its centre taken out is 'ky'.
'in'+'ky'='INKY'
'it's' acts as a link.
(Other definitions for inky that I've seen before include "Like writing fluid" , "Completely dark" , "Dark; stained" , "Jet" , "Pitch-black" .)