The most common and most likely answer to this crossword clue is the 4 plus 2 plus 9 letter word TREE OF KNOWLEDGE
I've seen this crossword clue in The Times
T
R
E
E
O
F
K
N
O
W
L
E
D
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E
More clues leading to the result TREE OF KNOWLEDGE
Buddy explains!
The straight part
We know that "a temptation that should have been avoided?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a temptation that should have been avoided? = tree of knowledge.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.