Soon time for a period of darkness (5)
I believe the answer is:
night
This clue could be a double definition.
'soon time' is the first definition.
'night' can be an answer for 'time' (I've seen this before). I'm not sure about the 'soon' bit.
'period of darkness' is the second definition.
(it is dark at night)
'for a' is the link.
Can you help me to learn more?
(Other definitions for night that I've seen before include "Illumination needed now" , "Time for bed" , "From dusk until dawn" , "Day's partner" , "Time between days" .)