Gloomy day on vessel (4)
I believe the answer is:
dark
'gloomy' is the definition.
('dark' can be similar in meaning to 'gloomy')
'day on vessel' is the wordplay.
'day' becomes 'd'.
'on' is a charade indicator (letters next to each other).
'vessel' becomes 'ark' (ark is a kind of vessel**).
'd'+'ark'='DARK'
(Other definitions for dark that I've seen before include "Shade of Oxford blue" , "Unlit" , "With little or no light" , "Such an unknown horse" , "Swarthy" .)