Staying out when it's very dark (7)
I believe the answer is:
stygian
'dark' is the definition.
(similar in meaning)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for stygian that I've seen before include "Dark and gloomy, like the mythological river Styx" , "underworld runner" , "Gloomy or murky after the river Styx" , "Very dark (poetic)" .)