Cathedral recess (4)
I believe the answer is:
apse
'cathedral recess' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for apse that I've seen before include "Semicircular recess in church" , "Part of a church with the altar" , "Large recess in church" , "Peas (anag) -- part of a church" , "Projection to the east end of a church" .)