Unusual trial involving universal custom (6)
I believe the answer is:
ritual
'custom' is the definition.
(ritual is a kind of custom)
'unusual trial involving universal' is the wordplay.
'unusual' is an anagram indicator.
'involving' is an insertion indicator.
'universal' becomes 'u'.
'trial' is an anagram of 'rital'.
'rital' placed around 'u' is 'RITUAL'.
(Other definitions for ritual that I've seen before include "Stereotyped behaviour" , "Ceremony, regular procedure" , "Customary observance or practice" , "Series of actions used in religious ceremony" , "Ceremony or protocol" .)