The hours of darkness (5)
I believe the answer is:
night
'the hours of darkness' is the definition.
(dark time of day)
This is the entire clue.
(Other definitions for night that I've seen before include "See 8" , "Time of darkness" , "''Twas the ... before Christmas, when all through the house / Not a creature was stirring, not even a mouse''" , "Sunset to sunrise" , "Saturday -- Fever (film)" .)