The most common and most likely answer to this crossword clue is the 12 letter word DISINHERITED
I've seen this crossword clue in The Daily Telegraph
D
I
S
I
N
H
E
R
I
T
E
D
Buddy explains!
The straight part
We know that "Not allowed to succeed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not allowed to succeed = disinherited.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 DISINHERITED mean?
DISINHERITED verb- To exclude from inheritance; to disown.