Grave, serious (4)
I believe the answer is:
grim
I believe this clue is a double definition.
'grave' is the first definition.
(thesaurus)
'serious' is the second definition.
(thesaurus)
(Other definitions for grim that I've seen before include "Serious; dour" , "Stern, cruel" , "Severe; horrible" , "Dire" , "Dreadful" .)