Solemnity (8)
I believe the answer is:
gravitas
'solemnity' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gravitas that I've seen before include "Seriousness - sobriety" , "Dignity; air of seriousness" , "Serious demeanour" , "Dignified manner/presence" , "Formal bearing" .)