Gravest

6 clues
Most sober
Least merry
Most somber
Most serious
Most serious and somber
Like a worst-case scenario
Similar answers:
Absolutegravity
Academicgrades
Academicprogram
Acowoncegrazed
Actofgrace
Adamsgrandson
Adayatthegraces
Adhesivestrip
Aerialphotograph
Aerogram
Againstgrain
Againstthegrain
Aggrade
Aggrades
Aggrandize
Aggravate
Aggravated
Aggravatedasalt
Aggravates
Aggravating