Part of a church with the altar (4)
I believe the answer is:
apse
'part of a church with the altar' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for apse that I've seen before include "Church recess in lapsed condition" , "Primate holds saint to be a" , "where praetor sat" , "Semicircular end of a church, behind the altar" , "Recess at east end of church" .)