Bishop's seat in ancient churches (4)
I believe the answer is:
apse
'bishop's seat in ancient churches' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for apse that I've seen before include "Church area" , "Rounded end of building" , "Recess at eastern end of a church" , "space for altar" , "room for an altar" .)