It will happen once this month, by chance not in church. (8)
I believe the answer is:
instance
'it will happen once this month' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'by chance not in church' is the wordplay.
'by' says to put letters next to each other.
'chance not' becomes 'stan' (I am not sure about this - if you are sure you should believe this answer much more).
'church' becomes 'CE' (abbreviation for Church of England).
'stan' put after 'in' is 'instan'.
'instan'+'ce'='INSTANCE'
Can you help me to learn more?
(Other definitions for instance that I've seen before include "Cite as an example" , "Case or example from ancients" , "Case - isn't acne (anag)" , "Particular example" , "A case or example of something" .)