Swore what sinner did on the Bible (7)
I believe the answer is:
averred
'swore' is the definition.
('aver' can be a synonym of 'swear')
'did on the bible' is the wordplay.
'did' becomes 'erred' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'the bible' becomes 'AV' (Authorised Version of the Bible).
'erred' put after 'av' is 'AVERRED'.
'what sinner' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for averred that I've seen before include "Held" , "Stated, asserted" , "Asserted to be the case" , "Claimed" , "Maintained" .)