Second attack when night becomes even darker? (7)
I believe the answer is:
moonset
'when night becomes even darker?' is the definition.
(the setting of the moon)
'second attack' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'attack' becomes 'onset' (synonyms).
'mo'+'onset'='MOONSET'
(Other definitions for moonset that I've seen before include "Going down of Earth's satellite" , "disappearance of satellite" , "When Earth's satellite disappears below the horizon" , "when night becomes even darker?" , "astronomical activity" .)