The most common and most likely answer to this crossword clue is the 9 letter word NEGLECTED
I've seen this crossword clue in The Daily Telegraph
N
E
G
L
E
C
T
E
D
More clues leading to the result NEGLECTED
Buddy explains!

The straight part
We know that "Forgotten" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case forgotten = neglected.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 NEGLECTED mean?
NEGLECTED verb- To fail to care for or attend to something.
- To omit to notice; to forbear to treat with attention or respect; to slight.
- To fail to do or carry out something due to oversight or carelessness.
- Suffering from neglect.