Parishioners learning about silly nonsense (8)
I believe the answer is:
folderol
'silly nonsense' is the definition.
(folderol means fuss or nonsense)
'parishioners learning about' is the wordplay.
'parishioners' becomes 'fold' (fold can mean a group of parishioners).
'learning' becomes 'lore' (both can mean tradition or knowledge).
'about' says the letters should be written in reverse.
'lore' reversed gives 'erol'.
'fold'+'erol'='FOLDEROL'
(Other definitions for folderol that I've seen before include "trifle" , "something unimportant" , "Ornamental object of no great value" , "Refrain" , "Trivial or nonsensical fuss" .)