Nearly dark, but not quite (4)
I believe the answer is:
nigh
'not quite' is the definition.
(thesaurus)
'nearly dark' is the wordplay.
'nearly' means to remove the last letter.
'dark' becomes 'night' (night is a kind of dark).
'night' with its final letter taken away is 'NIGH'.
'but' acts as a link.
(Other definitions for nigh that I've seen before include "Approaching" , "Near (old use)" , "Close at hand" , "Near; close" , "old touch" .)