Worship of the Devil disguised as Matins! (8)
I believe the answer is:
satanism
'worship of the devil' is the definition.
(I've seen this before)
'disguised as matins' is the wordplay.
'disguised' is an anagram indicator.
'as'+'matins'='asmatins'
'asmatins' with letters rearranged 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" .)