Hours of darkness are nearly without end (5)
I believe the answer is:
night
'hours of darkness' is the definition.
(dark time of day)
'nearly without end' is the wordplay.
'nearly' becomes 'nigh' (synonyms).
'end' suggests the final letters.
The final letter of 'without' is 't'.
'nigh'+'t'='NIGHT'
'are' acts as a link.
(Other definitions for night that I've seen before include "Opposite of day" , "See 1 Across" , "'The 2 8 of the dog in the _____'" , "See 8" , "8d [DARKNESS]" .)