The most common and most likely answer to this crossword clue is the 9 letter word INVISIBLE
I've seen this crossword clue in Financial Times
I
N
V
I
S
I
B
L
E
More clues leading to the result INVISIBLE
- Like the prime number eluding da Vinci, extremely hidden
- Belief that ink might be this in thing?
- Unable to part, detective leaves unseen
- Live in bliss every second — unseen, in disguise and inconspicuous
- Perhaps sister’s wearing veil in order not to be seen
- At home, fancy veil is keeping bride’s head impossible to see
Buddy explains!
The straight part
We know that "not to be seen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not to be seen = invisible.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 INVISIBLE mean?
INVISIBLE noun- An invisible person or thing; specifically, God, the Supreme Being.
- A Rosicrucian; so called because avoiding declaration of his craft.
- One of those (as in the 16th century) who denied the visibility of the church.
- To make invisible, to invisiblize.
- Unable to be seen; out of sight; not visible.
- Not appearing on the surface.
- Apparently, but not actually, offline.
- That is ignored by a person.