Superlatively gloomy day ship set out (7)
I believe the answer is:
darkest
'superlatively gloomy' is the definition.
'day ship set out' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'ship' becomes 'ark' (I've seen this in other clues).
'out' indicates an anagram (out can mean wrong or inaccurate).
'set' anagrammed gives 'est'.
'd'+'ark'+'est'='DARKEST'
(Other definitions for darkest that I've seen before include "Most devoid of light" , "Most gloomy" , "'The . . . . . . . hour comes before the dawn'" , "Gloomiest or blackest" , "Most dim" .)