In Cornwall, Catholic assumes a belief that can’t be questioned (6,3)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

C (Catholic), inside A CREDO=”a belief”; all in SW (“In Cornwall”, which is in the SW/south-west of England) “assumes” as in ‘takes on’/’bears’/’wears’ to indicate insertion of C into A CREDO