Perhaps every church includes one (4)
I believe the answer is:
apse
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'perhaps every church includes' is the wordplay.
'includes' indicates the answer is hidden within the clue.
'APSE' is hidden within 'perhaps every church'.
Can you help me to learn more?
(Other definitions for apse that I've seen before include "Rounded end of building" , "Church recess with domed or arched roof" , "Recess at east end of church" , "Large recess in church" , "Architectural feature in a church" .)