Nonsense is the originally crude Catholic label (6)
I believe the answer is:
papist
'crude catholic label' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'nonsense is the originally' is the wordplay.
'nonsense' becomes 'pap' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'originally' indicates taking the first letters.
The initial letter of 'the' is 't'.
'pap'+'is'+'t'='PAPIST'
Can you help me to learn more?
(Other definitions for papist that I've seen before include "supporter of Rome" , "Scornful word for Roman Catholic sometimes heard in N Ireland" , "Believer rudely" , "Catholic offensively" , "supporter of Catholicism" .)