Solemn practice (4)
I believe the answer is:
rite
'solemn practice' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rite that I've seen before include "Established religious ceremony" , "Customary observance" , "(Words for) ceremony" , "Ceremony - ritual" , "(Words used in) ceremonial" .)