Dark period (5)
'dark period' is the definition.
(period of the day when it's dark)
This is all the clue.
(Other definitions for night that I've seen before include "Thing (anag)" , "Illumination needed now" , "From dusk until dawn" , "Darkness; see you in the morning" , "Time for bed" .)
(Other definitions for outage that I've seen before include "When one lacks power" , "Power cut" , "time of no power" , "Temporary suspension of operation" , "loss of power" .)