The most common and most likely answer to this crossword clue is the 9 letter word FORGOTTEN
I've seen this crossword clue in The Daily Telegraph
F
O
R
G
O
T
T
E
N
More clues leading to the result FORGOTTEN
- Left behind, eschew tea, as we hear, before bedtime?
- Do without lost tent that’s been left behind
- Out of one’s mind
- Do without tent carelessly left behind
- Neglected grotto built in marshy area
- Omitted all bar one of the team and lost
- Achieved 50 per cent of score after pro not recalled
- Lost in mist -- bad when two characters swap places
- Catch up after pro got left behind
- Out of one’s mind?
Buddy explains!

The straight part
We know that "lost" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lost = forgotten.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 FORGOTTEN mean?
FORGOTTEN verb- To lose remembrance of.
- To unintentionally not do, neglect.
- To unintentionally leave something behind.
- To cease remembering.
- Euphemism for fuck, screw (a mild oath).
- A person or thing that has been forgotten.
- Of which knowledge has been lost; which is no longer remembered.