Almost dark, but not quite (4)
I believe the answer is:
nigh
'not quite' is the definition.
(thesaurus)
'almost dark' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'dark' becomes 'night' (night is a kind of dark).
'night' with its last letter taken off is 'NIGH'.
'but' acts as a link.
(Other definitions for nigh that I've seen before include "See 23" , "in the vicinity?" , "Close at hand" , "Almost (arch.)" , "Near; close" .)