The most common and most likely answer to this crossword clue is the 8 letter word VANISHES
I've seen this crossword clue in Financial Times
V
A
N
I
S
H
E
S
Buddy explains!
The straight part
We know that "suddenly can’t be seen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case suddenly can’t be seen = vanishes.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 VANISHES mean?
VANISHES verb- To become invisible or to move out of view unnoticed.
- To become equal to zero.
- To disappear; to kidnap