In Gt Manchester town one I miscast badly for churchiness (15)

'in gt manchester town' is the definition.
I can't tell whether this definition defines the answer.

'one i miscast badly for churchiness' is the wordplay.
'one' becomes 'i' (Roman numeral).
'badly' indicates an anagram.
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'churchiness' becomes 'eccles' (I can't explain this - if you can you should give a lot more credence to this answer).
'i'+'miscast'='imiscast'
'imiscast' anagrammed gives 'asticism'.
'i'+'asticism'='iasticism'
'iasticism' after 'eccles' is 'ECCLESIASTICISM'.