Satanists' leader breaks mirror in church recess (4)
I believe the answer is:
apse
'church recess' is the definition.
(part of a church)
'satanists' leader breaks mirror' is the wordplay.
'leader' suggests taking the first letters.
'breaks' is an insertion indicator.
'mirror' becomes 'ape' (I've seen this in another clue).
The first letter of 'satanists' is 's'.
's' put inside 'ape' is 'APSE'.
'in' is the link.
(Other definitions for apse that I've seen before include "Where you could find a magistrate's chair" , "part of cathedral" , "place in church" , "Architectural feature in a church" , "Recess at east end of church" .)