Approaching tense hours of darkness (5)
I believe the answer is:
Here is my best explanation:
'hours of darkness' is the definition.
(I've seen this before)
'approaching tense' is the wordplay.
'approaching' becomes 'nigh' (I've seen this before).
'tense' becomes 't' (abbreviation used in many dictionaries).
(Other definitions for night that I've seen before include "''Twas the ... before Christmas, when all through the house / Not a creature was stirring, not even a mouse''", "From sunset to sunrise", "evening", "That's when it's darkest", "Sunset to sunrise", "Saturday -- Fever (film)", "For most, the time for sleep", "period of gloom", "- School, - owl" and "What proverbially follows day".)