Semicircular recess (4)
I believe the answer is:
apse
'semicircular recess' 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 "room for an altar" , "Recess in church wall" , "Domed section of a church" , "space for altar" , "Semicircular end of a church, behind the altar" .)