The most common and most likely answer to this crossword clue is the 6 plus 6 letter word OBJECT LESSON
I've seen this crossword clue in The Sunday Telegraph
O
B
J
E
C
T
L
E
S
S
O
N
More clues leading to the result OBJECT LESSON
- Don't complain so much about instructive example
- Refuse to agree reading in church is instructive experience
- Don’t mind so much running — it’s a good example
- Salutary warning
- Complain more flesh visible? That’s a warning
- Make fewer complaints about practical demonstration
- On leaving Jobcentre, loses working experience of instructive nature
- For eample, a thing read by the vicar
- Disagree having fewer commitments is example to follow
- Protest by class, a convincing demonstration
Buddy explains!
The straight part
We know that "a warning" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a warning = object lesson.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.