The most common and most likely answer to this crossword clue is the 8 letter word MORALISE
I've seen this crossword clue in The Guardian
M
O
R
A
L
I
S
E
More clues leading to the result MORALISE
Buddy explains!

The straight part
We know that "Lecture" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lecture = moralise.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 MORALISE mean?
MORALISE verb- To make moral reflections (on, upon, about or over something); to regard acts and events as involving a moral.
- To say (something) expressing a moral reflection or judgment.
- To render moral; to correct the morals of; to give the appearance of morality to.
- To give a moral quality to; to affect the moral quality of, either for better or worse.
- To apply to a moral purpose; to explain in a moral sense; to draw a moral from.
- To supply with moral lessons, teachings, or examples; to lend a moral to.