Extremely serious (6)
I believe the answer is:
solemn
'extremely serious' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for solemn that I've seen before include "Formal, dignified" , "Formal, ceremonius" , "Serious and formal" , "Portentous" , "Serious and humourless" .)