The most common and most likely answer to this crossword clue is the 9 letter word PASSER-BY
P
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S
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B
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More clues leading to the result PASSER-BY
- Observer employing a Serb spy
- After review Serb pays possible witness
- Faded second of drives, next to spectator
- Potential witness placing king in “old hat”
- Yard’s pursuing press about keeping a note for witness
- Observer and Times supporting old-fashioned Republican
- Transmit series made for TV Times and Observer?
Buddy explains!

The straight part
We know that "Observer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case observer = passer-by.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 PASSER-BY mean?
PASSER-BY noun- A person who is passing by (that is, walking past).