A religious mystery round the city (10)
I believe the answer is:
sacramento
'city' is the definition.
(Sacramento is an example)
'a religious mystery round the' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'round' could be 'o' (round can mean a round object) and 'o' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sacramento that I've seen before include "State capital of California" , "port" , "US city" .)