Architectural projection. Sometimes ecclesiastical? Primarily! (4)
I believe the answer is:
apse
'architectural projection' is the definition.
(I've seen this in another clue)
'sometimes ecclesiastical? primarily' is the wordplay.
'sometimes' becomes 'aps' (I can't explain this - if you can you should give a lot more credence to this answer).
'primarily' says to take the initial letters.
The first letter of 'ecclesiastical' is 'e'.
'aps'+'e'='APSE'
Can you help me to learn more?
(Other definitions for apse that I've seen before include "Part of a church with the altar" , "Rounded end of building" , "Church's part" , "Church recess with domed or arched roof" , "Large recess in church" .)