Approaching tense hours of darkness (5)
I believe the answer is:
night
'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).
'nigh'+'t'='NIGHT'
(Other definitions for night that I've seen before include "See 1 Across" , "Time for turning" , "'Red sky at . . . . ., shepherd's delight'" , "What proverbially follows day" , "Dark hours" .)