Dark, rough sea, about to go to bed (9)
I believe the answer is:
saturnine
'dark' is the definition.
(thesaurus)
'rough sea about to go to bed' is the wordplay.
'rough' indicates an anagram.
'about' is an insertion indicator.
'to go to bed' becomes 'turnin' ('turn in' can be a synonym of 'go to bed').
'sea' is an anagram of 'sae'.
'sae' enclosing 'turnin' is 'SATURNINE'.
(Other definitions for saturnine that I've seen before include "A rust nine chnage to be grave and gloomy" , "dull" , "Gloomy (temperament)" , "shining? The opposite" , "offering little amusement" .)