The most common and most likely answer to this crossword clue is the 10 letter word EVANGELISE
I've seen this crossword clue in The Guardian
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Buddy explains!

The straight part
We know that "to give a sermon" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to give a sermon = evangelise.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does EVANGELISE mean?
EVANGELISE verb- To tell people about (a particular branch of) Christianity, especially in order to convert them; to preach the gospel to.
- To preach any ideology to those who have not yet been converted to it.
- To be enthusiastic about something, and to attempt to share that enthusiasm with others; to promote.