Dark period almost close (4)
I believe the answer is:
Here is my best explanation:
'close' is the definition.
('nigh' can be similar in meaning to 'close')
'dark period almost' is the subsidiary indication.
'dark period' becomes 'night' (I've seen this before).
'almost' means to remove the last letter.
'night' with its last letter taken off is 'nigh'.
(Other definitions for nigh that I've seen before include "Near at hand", "close at hand", "'Almost, near (4)'", "Nearly", "nearby", "Near in time or place", "old touch", "Nearby", "in the vicinity?" and "Old word for near, as the end may be".)