Quotation from what was once a sermon (7)
I believe the answer is:
extract
'quotation' is the definition.
(an extract is quotation or selection from a text)
'once a sermon' is the wordplay.
'once' becomes 'ex' (prefix meaning former or onetime).
'a sermon' becomes 'tract' (I can't justify this - if you can you should give a lot more credence to this answer).
'ex'+'tract'='EXTRACT'
'from what was' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for extract that I've seen before include "Pull out - quotation" , "Draw or pull out" , "Short passage from a longer work" , "essence" , "Withdraw - quotation" .)