The most common and most likely answer to this crossword clue is the 8 letter word SENTINEL
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More clues leading to the result SENTINEL
- Guardsman told to attack the Spanish
- Given mission to enter cell, avoiding outside guard
- Submitted by the Spanish guard
- Posted home, the Spanish guard
- Guard dispatched before brief communist uprising
- Head of security sacked lenient guard
- Guard despatched to breached line
- Guard dispatched one with a half-nelson
- Deposit left to secure new metal guard
- Defender playing table tennis, discarding broken bat
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 = sentinel.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 SENTINEL mean?
SENTINEL noun- A sentry, watch, or guard.
- A private soldier.
- A unique string of characters recognised by a computer program for processing in a special way; a keyword.
- A sentinel crab.
- A sign of a health risk (e.g. a disease, an adverse effect).
- To watch over as a guard.
- To post as guard.
- To post a guard for.