It's dark almost on time (5)
I believe the answer is:
Here is my best explanation:
'it's dark' is the definition.
(night is a kind of dark)
'almost on time' is the subsidiary indication.
'almost' becomes 'nigh' ('nigh' can be a synonym of 'almost').
'on' is a charade indicator (letters next to each other).
'time' becomes 't'.
(Other definitions for night that I've seen before include "Saturday -- Fever (film)", "Time for bed", "period of gloom", "Time for turning", "Evening", "Darkness; see you in the morning", "Time of darkness", "See 9", "Dusk to dawn" and "Sunset to sunrise".)