Crossword Genius

Architectural projection. Sometimes ecclesiastical? Primarily! (4)

Ross

I believe the answer is:

apse

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Everyman puzzle in The Observer.
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