The most common and most likely answer to this crossword clue is the 9 letter word UNNOTICED
I've seen this crossword clue in The Times
U
N
N
O
T
I
C
E
D
Buddy explains!
The straight part
We know that "not in the view of The Observer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not in the view of the observer = unnoticed.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 UNNOTICED mean?
UNNOTICED adjective- Not noticed.