Chapel part (4)
I believe the answer is:
apse
'chapel part' 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" , "Peas (anag) - found in church" , "where praetor sat" , "Peas (anag) -- part of a church" , "Offshoot from the choir, nave or aisle in a church" .)