Some are solemn (5)
I believe the answer is:
oaths
'some are solemn' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oaths that I've seen before include "Curses - words of honour?" , "professions of loyalty" , "Cursing" , "Profane expressions" , "truths may be established by them" .)