At Mass, in perverted rite? (8)
I believe the answer is:
satanism
'rite?' is the definition.
(satanism is a religious practice)
'at mass in perverted' is the wordplay.
'perverted' indicates anagramming the letters.
'at'+'mass'+'in'='atmassin'
'atmassin' anagrammed gives 'SATANISM'.
(Other definitions for satanism that I've seen before include "Belief in and reverence for the devil" , "Devil-worship" , "Worship of the devil using mantissa" , "Evil praying" .)