Gloomy day on rescue vessel (4)
I believe the answer is:
dark
'gloomy' is the definition.
(I know that gloomy can be written as dark)
'day on rescue vessel' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'rescue vessel' becomes 'ark' (rescue ship in the Bible).
'd'+'ark'='DARK'
(Other definitions for dark that I've seen before include "Sombre in colour" , "Unlit" , "Sinister - evening" , "Opposite of light" , "Absence of light" .)