Unilluminated vessel on Dee (4)
I believe the answer is:
dark
'unilluminated' is the definition.
('dark' can be similar in meaning to 'unilluminated')
'vessel on dee' is the wordplay.
'vessel' becomes 'ark' (ark is a kind of vessel**).
'on' is a charade indicator (letters next to each other).
'dee' becomes 'd'.
'ark' put after 'd' is 'DARK'.
(Other definitions for dark that I've seen before include "lacking lights" , "Opposite of light" , "Shade of Oxford blue" , "Black, perhaps - night" , "Closed as theatre" .)