The most common and most likely answer to this crossword clue is the 8 letter word OMISSIVE
I've seen this crossword clue in The Times
O
M
I
S
S
I
V
E
More clues leading to the result OMISSIVE
Buddy explains!
The straight part
We know that "prone to leave things out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case prone to leave things out = omissive.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.