The most common and most likely answer to this crossword clue is the 10 letter word DISAPPEARS
I've seen this crossword clue in Independent
D
I
S
A
P
P
E
A
R
S
More clues leading to the result DISAPPEARS
Buddy explains!

The straight part
We know that "becomes lost" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case becomes lost = disappears.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 DISAPPEARS mean?
DISAPPEARS verb- To vanish.
- To make vanish; especially, to abduct and murder surreptitiously for political reasons.
- To go away; to become lost.