Satanic leader breaks mirror in church recess (4)
I believe the answer is:
apse
'church recess' is the definition.
(I've seen this in another clue)
'satanic leader breaks mirror' is the wordplay.
'leader' indicates taking the first letters.
'breaks' indicates putting letters inside.
'mirror' becomes 'ape' (I've seen this in another clue).
The first letter of 'satanic' is 's'.
's' going within 'ape' is 'APSE'.
'in' acts as a link.
(Other definitions for apse that I've seen before include "Where you could find a magistrate's chair" , "Where the altar is usually found in a church" , "Large recess in church" , "Church area" , "Peas (anag) -- part of a church" .)