Dark period almost close (4)
I believe the answer is:
nigh
'close' is the definition.
('nigh' can be similar in meaning to 'close')
'dark period almost' is the wordplay.
'dark period' becomes 'night' (I've seen this before).
'almost' means to remove the last letter (most of the word but not all of it).
'night' with its last letter taken away is 'NIGH'.
(Other definitions for nigh that I've seen before include "Near in time or place" , "Near, as in archaic usage" , "See 23" , "All but one" , "Adjacent" .)