Serious plot in churchyard (5)
I believe the answer is:
grave
'serious' is the definition.
(both can mean solemn)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for grave that I've seen before include "Grim-faced" , "Sober" , "Burial site" , "Weighty" , "Hazardous" .)