Altar area (4)
I believe the answer is:
apse
'altar area' 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 "Recess at eastern end of a church" , "Church recess with domed or arched roof" , "Part of a church with the altar" , "where praetor sat" , "Architectural feature in a church" .)