Cathedral nook (4)
I believe the answer is:
'cathedral nook' 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 "place in church" , "Peas (anag) - found in church" , "Offshoot from the choir, nave or aisle in a church" , "Recess with domed rof in a church" , "Primate holds saint to be a" .)