Stygian vessel following dead (4)
I believe the answer is:
dark
'stygian' is the definition.
('dark' can be similar in meaning to 'stygian')
'vessel following dead' is the wordplay.
'vessel' becomes 'ark' (ark is a kind of vessel**).
'following' says to put letters next to each other.
'dead' becomes 'd'.
'ark' after 'd' is 'DARK'.
(Other definitions for dark that I've seen before include "Deprived of light" , "Gloomy" , "lacking lights" , "Devoid of light" , "Lacking light" .)