Serious, gloomy (4)
I believe the answer is:
grim
I believe this clue is a double definition.
'serious' is the first definition.
(thesaurus)
'gloomy' is the second definition.
(I know that gloomy can be written as grim)
(Other definitions for grim that I've seen before include "Very serious/gloomy" , "looking black" , "Dour and severe" , "Harsh and merciless" , "Serious; dour" .)