The most common and most likely answer to this crossword clue is the 15 letter word MIGHT-HAVE-BEEN
I've seen this crossword clue in The Times
M
I
G
H
T
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H
A
V
E
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B
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N
Buddy explains!
The straight part
We know that "Unfulfilled person" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unfulfilled person = might-have-been.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 MIGHT-HAVE-BEEN mean?
MIGHT-HAVE-BEEN noun- Someone or something whose potential greatness was not achieved.